Hi ehuesman,
You are inside the range of fine tuning and smaller or more subtle changes where it’s really impossible to use specs (either yours or the materials) to predict how the changes you are considering will affect you with any certainty because the unknowns and variables including differences in body types and weight distribution along with sleeping positions, physiology, and individual sensitivities can be an even bigger factor than the changes themselves. When you are in this position then your own “educated best judgement” based on your own personal experience would probably be more accurate than anything someone else could suggest to you.
There isn’t a specific definition of progressive or differential layering and these are just concepts or terms I use to clarify different types of approaches to mattress design. While it doesn’t really matter which one it is … yours would be more of a “progressive” approach where you have the firmest layer on the bottom and the layers become progressively softer as you move to the top. A design that I would call more “differential” would be a two layer mattress with something like a 6" firm core with a much softer 3" comfort layer on top of it with no transition layer in between them so there would be a much bigger differential in the ILD’s of the two layers.
Support is really a very vague term and it really depends on what part of the body you are talking about “supporting”. A hard floor for example would have poor support overall because it wouldn’t support the inward curves of the human body and would only “support” the contact points (hips and shoulders) so the rest of the body would “sag” down to the floor because there was nothing underneath them to help hold them up. I would keep in mind that a mattress that is suitable in terms of alignment needs to “stop” some parts of the body from sinking down too far (such as the hips) and “allow” other parts to sink in more (such as the shoulders) and also “fill in” all the gaps in the body profile. There is more about primary and secondary support and their relationship to pressure relief in post #4 here. There is also more information in post #2 here that talks about the various “symptoms” that people may experience on a mattress and some of the possible causes for them that can help with the “detective work” and trial and error that may be necessary to reach your ideal combination of layers.
Deep support (or what I call “primary support”) that stops the heavier parts of the body from sinking down too far comes primarily from the deeper layers. What I call secondary support is the support that fills in the recessed gaps of the body and is primarily the function of the upper layers (along with pressure relief). Middle or transition layers will help with and affect both. The need for some parts of the body to sink in more easily into softer layers for pressure relief and secondary support and for other parts to be “stopped” from sinking too far by firmer layers for better alignment are conflicting and the challenge is always to find the best balance between them. Comfort layers that are too thick/soft can be risky for alignment so the “safest” approach is usually “just enough” in terms of thickness and softness to relieve pressure in your most pressure prone sleeping positions so that you have the least possible risk of alignment issues. “Just enough” can vary widely between people.
There are too many variables and unknowns and I don’t have enough reference points of combinations you have slept on for me to know but the difference in ILD would be relatively small and I’m not sure I would have any confidence in a weight based calculation that tried to determine ILD with the specificity you are looking for. Most people wouldn’t feel a difference of 2 or 3 ILD anyway and ILD also isn’t consistent across the surface of the layer (it varies from spot to spot by several ILD).
Having said that … if your layer really was 24 ILD and you changed to a layer that was 17 ILD which is a 7 ILD difference (and again it wouldn’t be possible to be that specific) then most people would likely notice a difference but there would be no way to know if it was “enough” difference until you slept on it.
If I was in your shoes and since you can’t return your current layer anyway I would probably consider adding a topper where you can work with both the the thickness and the ILD of your upper softer layers rather than only working within the limitations of changing a 3" layer.“act” firmer than if the same layer was Talalay and softening up this layer would also make a difference in pressure relief.
I don’t know but “just enough” thickness and softness would probably have the least possible risk for alignment issues so I would tend to avoid the softest ILD’s in thicker layers at your weight unless it was clear that there was no other alternative.
Again the goal is alignment (and the different levels of support under different parts of your body is just a means to an end) but every change to pressure relief will also have a corresponding effect on support (you need to sink in more to improve pressure relief and sink in less to improve support) so the key is finding the balance between them where both are inside the range that is suitable for you and the mattress also has the “feel” that you prefer (and feel is different again from either pressure relief and support).
If you do decide to go in the direction of adding a topper then post #2 here and the topper guidelines it links to has more information that can help you use your sleeping experience as a reference point for choosing the thickness and softness of a topper that has the best chance of working well for you.
Phoenix