Hi canadian,
You’re right that there are really too many unknowns, variables and personal preferences involved for anyone to be able to know the combination of layers that will work best for someone else with any certainty based on specs (either yours or a mattress) or “theory at a distance” … especially if you are more “sensitive” and without having any specific reference points of knowing your experience on similar mattresses (with known specs) that you have tested or slept on (see mattress firmness/comfort levels in post #2 here).
If you are attracted to the idea of designing and building your own mattress out of separate components and a separate cover then the first place I would start is by reading option 3 in post #15 here and the posts it links to so that you have realistic expectations and that you are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process. While it can certainly be a rewarding project … the best approach to a DIY mattress is a “spirit of adventure” where what you learn and the satisfaction that comes from the process itself is more important than any cost savings you may realize (which may or may not happen).
I would also keep in mind that there is no specific definition of progressive or differential constructions and these are only generic concepts that may have little relevance on an individual basis and that neither one is nearly as important as whether a mattress is a good match for you in terms of PPP and to know this with any certainty you would either need to test a mattress in person or sleep on it regardless of whether it was closer to a more complex progressive design with multiple layers or a differential design with only two layers (or even a mattress that used a single layer of material).
If you do decide to go with a DIY approach then the most effective strategy would be to use a known reference point of a mattress with a known design that you have confirmed is a good match for you in terms of PPP and duplicate the type and blend, the thickness, and the firmness of each layer as closely as possible and use a very similar cover.
If that’s not possible then I would take a “bottom up” approach and work towards your ideal design one incremental step at a time because the choice of your support core will have a significant effect on the thickness and firmness of the comfort layers on top of it that will be suitable for you. Since you appear to prefer a firmer mattress you could start with just a 6" firm core (or even a medium/firm with your lighter weights) and then use your actual experience on the core alone along with post #2 here and the topper guidelines it links to as a reference point to help you decide on the thickness and firmness of the next layer you wish to add to your design. I would also tend to err on the side of firmness with each layer so that you don’t end up with a combination that is too soft for you which is much more difficult to “fix” without returning or exchanging a layer. You could then sleep on these two layers and once again use your actual sleeping experience to decide whether you needed an additional layer. Once you have the layers you are comfortable with then the final step would be to decide on and buy the type of cover that you would prefer and that tightly fits the thickness of your latex layers to finish your mattress.
I would also make sure that you have a good return/exchange policy on each of the layers you purchase to reduce the risk and cost of any mistakes you make so that you aren’t “stuck” with a layer that doesn’t work well for you. Without this you would be taking a more risky (or costly) approach.
Just in case you haven’t seen it … there are some options in post #4 here that may give you the chance to test some latex combinations locally that may be helpful.
Post #21 here also includes some of the better online sources I’m aware of that ship mattresses, toppers, or individual layers across Canada so you can check to see which of them sell individual layers that you are interested in and compare the costs and check to make sure that you are comfortable with the return or exchange policies for each of them as well.
Phoenix