Hi jkozlow3,
ILD isn’t “additive” in this way. Different people will “feel” more of the firmness of some layers than others. Most people will “feel” the firmness/softness of the top layers more than the bottom layers but the thickness of the upper layers that will make the most difference for a particular person will also depend on their body type.
For many people (including me) your second example would likely feel softer regardless of how you may experience it but it will also depend on how much the layers are “bottoming out” and for some people having the 28 ILD on the bottom would reduce the effect of the substrate more and could feel softer.
An example would be to test a PLB mattress on a firm foundation and then test the same mattress on their latex foundation which has 4" of latex and you would find that the latex foundation would make the mattress feel softer.
Perhaps a better illustration yet would be if you were to to replace the top layer of both of your examples with a layer that was the same ILD but was an inch thicker they would both feel softer.
This just isn’t accurate and you can’t “remove” the effect of other layers or substrates because a topper is always used on top of something else.
While I do understand your point and there are some similar comments based on “rough math” in post #26 here and post #2 here that go into great detail … your examples are more about mattress design than topper choices and using your comments as a reliable indicator in “real life” to choose a topper would be fairly misleading for most people.
Phoenix