Hi vjane89,
This is the part of mattress shopping that I can’t help with because there are too many variables, unknowns, and individual preferences to use “theory at a distance” to choose a mattress and only you can feel what you feel on a mattress (see mattress firmness/comfort levels in post #2 here). There is no “right” amount for you to sink into a mattress because different people have different preferences for the softness of the comfort layer but what’s important is that no matter how far you sink into a mattress that your spine is in good alignment in all your sleeping positions. The guidelines in the tutorial post (there are 3 links there that can give you some testing suggestions and guidelines) are the best way I know to make sure your mattress testing is a reasonable approximation of what you will experience when you sleep on the mattress. I would also make sure that you spend “enough” time testing the mattress (a minimum of 15 minutes while you are completely relaxed) so you have some confidence that it’s a suitable choice for you. I would also make sure you know the options you have available if you need to do any “fine tuning” after a purchase but good testing will greatly increase your odds of success.
I would also keep in mind that “feel”, spinal alignment, and pressure relief are all very different from each other and that going by “feel” may have lower odds of success than random chance alone (see this study). There need to be different levels of “support” under different parts of the body and some areas where your mattress needs to “allow” you to sink in more deeply (less support) and some that the mattress needs to “stop” you from sinking in to deeply (more support) so the end result is that you are in good alignment in all your sleeping positions (or in your case on your back). You can see from this diagram how a mattress that is too firm can lead to sleeping out of alignment just as easily as a mattress that is too soft.
Phoenix