Hi TweedSAFD,
While it may not be the answer you may be hoping for … the only “real” answer is “just enough” to relieve pressure in all your sleeping positions. The thickness of any specific layer that works best is always relative to the rest of the mattress design. All the layers of a mattress interact with and affect all the other layers so the actual thickness of any layer that is “ideal” would depend on the specifics of all the other layers in the mattress and how that specific combination interacts with your body type, sleeping positions, and preferences. Post #2 here has links to some of the articles that discuss some of the theory and concepts behind layer thickness and ILD for different designs and constructions and a forum search on “critical zone depth cradle” (you can just click this) will bring up more posts that discuss the relationship of the depth of cradle or what I call the “critical zone” to layer thickness and ILD but these are only generic concepts and only your own testing or sleeping experience can tell you whether a specific design provides the best possible balance between comfort/pressure relief and support/alignment. There is no “theory” or formula that can answer this for any specific person without specific reference points based on personal experience but the guidelines can provide a good starting point for those who want to become more involved with the specs of their mattress design.
Post #14 here also has some information about the potential benefits of thicker layers or mattresses for higher weights that may be useful.
In most cases though … there are “many roads to Rome” (Rome being the mattress design that provides the ideal combination of pressure relief and alignment) and there are usually several different designs that can provide a similar outcome.
Phoenix