How do you custom choose latex layers?

I am asking this here and now because I’m too impatient to wait till tomorrow to phone a store and ask someone.

Savvy Rest lets you completely design your own layering system but… how, in god’s name does one know what to pick? I wouldn’t have a clue. In fact, other than a professional who works in the industry, how would anybody be able to make an informed choice? Savvy Rest allows for 3 layers… each of which has 8 firmnesses and 2 types of latex. Do they have 16 x 16 x 16 ( 4096) different combinations on hand to try? :S
But seriously… even if, for each layer, one chooses from 2 different types of material and only 3 levels of firmness that is still 512 possibilities. Can anyone explain how one would go about making that choice?