This should be an interesting project
Your height and weight certainly help (lower weights than average) but it would also help to know a little bit more about the types of mattresses in general that you have been most comfortable with to get a sense of your preferences as well. Do you like sinking in more deeply, do you like firmer (more on top of the mattress), do you tend towards pressure issues, do you have back issues. All of these and other guidelines from your history would help to narrow down the “best” theoretical construction for you.
What I have called “differential” and “progressive” constructions are really different pathways to the same end which is good pressure relief and alignment. The differences between them don’t have clearly defined “edges” but are more of a general approach to layering.
Each person needs a pressure relieving cradle which is deep enough and conforms to their body shape enough to relieve pressure points below their pressure tolerance. Lets say for the sake of argument that this cradle needs to be 3". This would mean that a 3" comfort layer that was soft enough to provide most of this pressure relieving cradle could use a much firmer support layer below it. A comfort layer that was only 2" in this case wouldn’t by itself provide a deep enough cradle and if the layer below it was too firm you would “go through” the top layer and your pressure points would be supporting too much weight on too small an area because of the firmer layer below it. While all the layers of a mattress compress simultaneously … it helps to visualize this to think of the layers compressing in a certain order (first the top, then the next etc). In a case like this … you would use a softer support layer to help form a deeper cradle. If the comfort layer was already soft and thick enough … then it wouldn’t need any help and the support layers could be as firm as you needed them to be for best alignment.
Typically a comfort layer will come in about 1" increments which means that in most cases you need to choose between 2" and 3". Adjusting middle or lower layers however can create finer adjustments. This is where the support factor comes in.
If you had 2" of 20 ILD foam on the top of a mattress and you needed an extra 1/2" of cradle then a 32 ILD layer (which needed 32 lbs to compress a 4" layer by 1") would only need less than 16 lbs to compress to 1/2". It would then take @ 46 lbs (30 + 16) to compress a further 1" (1.5" total). What this means is that a 2" layer of 20 ILD over 32 ILD foam would give you the depth of cradle you needed but beyond this it would be roughly as supportive as a 46 ILD support layer. This would be the very rough equivalent of 2.5" of the same type of comfort layer over 46 ILD foam. The first example would be more progressive and the second would be more differential but would end up with roughly the same pressure relief. Of course the math here is not exact but the idea is that they would be roughly equivalent.
Another way to put this would be to use a 4" layer of 15 ILD polyfoam with a 2.5 sag factor as an example. This foam would take 15 lbs of pressure for a 50 sq inch foot to compress it by 1". It would take 2.5 x 15 or 37.5 lbs to compress it by 65% (roughly 2.6"). This means that it would take an extra 22.5 lbs to compress it the extra 1.6". So this foam would take a little more than 7 lbs to compress 1/2" (compression curves are not usually linear and the initial compression curve is generally steeper than the subsequent compression curve with deeper compression), 15 lbs to compress 1", 22 lbs to compress 1.5", 29 lbs to compress 2", 36 lbs to compress 2.5" (the middle range of compression is more linear) and about 45-50 lbs to compress 3" (the deepest compression is also not linear and starts getting firmer even faster which could be called bottoming out). This means that an area around the hips that was exerting 50 lbs per 50 sq inches of pressure could compress the top 15 ILD layer 3" but a 50 ILD lower support layer by only an extra 1". The odds are good that by the time you have sunk into the mattress this deeply that the surface area of your body that was bearing weight would be less than 50 lbs per 50 sq inches. Of course the process of how each layer reacts is along a continuous spectrum rather than in “steps” as in this example and is further influenced by the thickness of the mattress as a whole but the idea is correct.
The reason that differential is “easier” but progressive can be “more accurate” is because differential allows you to choose each layer more for a single purpose (either pressure relief or support) without taking into consideration as much how they interact or influence each other. You can choose a top layer that you know is thick and soft enough for good pressure relief without worrying as much about whether you will feel a really firm support layer underneath it. The advantage of the progressive is that you can fine tune the layering more (you’re not limited by the thickness increments of the comfort layer materials). It can also use thinner comfort layers which can often allow the support layers to provide better support for multiple sleeping positions.
One other thing to bear in mind is that latex is usually tested for ILD with a 6" layer while polyfoam is tested for ILD with a 4" layer which means that the measurements don’t directly translate into each other. ILD taken with a thicker layer needs more pressure to compress it 25% because the 25% thickness is more than with a thinner layer. The net effect of this is that latex ILD’s will be slightly softer than the same ILD’s in polyfoam.
Having said all that (and I know that this can get quite complex) you have the benefit or working with a HR foam with a higher sag factor which means that you have more flexibility when it comes to choosing layers.
Subject to knowing your “history” and “preferences” … I would tend to start off in one of two directions depending on how “risky” you wish to be.
The first (differential) would use a 3" comfort layer which because of your lower weights and with the right firmness should give you enough thickness that you can choose a firmer support layer below it without worrying about feeling it. I would likely choose an ILD in the range of 15 - 20 (which would be a little firmer than latex in the same ILD). Under this I would put a firm 6" layer in the range of high 30’s or low 40’s. To save money you could also use HD foam in the support layer although HR would work a little better (because it’s support factor would be higher and it would “hold up” your heavier parts better). Average ILD (assuming 15 ILD top and 40 ILD bottom) would be @32
The second approach would use the same ILD foam in the top layer but it would be thinner say 2". I would put this over a 3" “transition” layer in the high 20’s to low 30’s (to give you a deeper cradle but also better support once you reached the depth of cradle you needed) and then a lower layer of another 4" or so of very firm (@ 48). Average ILD (assuming 15 over 28 over 48) would be @ 34.
The unknown about either of these would be your “critical zone” which is the effective depth of cradle that is best for you and that provides enough pressure relief for your sensitivity and of course your preferences and YMMV.