Hi Shostie,
Post #2 here has links to some of the “theory” about layer thickness and firmness levels but the most effective way to choose is either with your own personal testing or if that isn’t possible with an online purchase then a more detailed conversation with the manufacturer who will know more about their mattresses than anyone else and who will have a set of standard suggestions for different body types and sleeping styles based on their own experience and knowledge (that may be modified based on your testing) and the “averages” of their customers.
While I don’t have personal experience with this (although there are forum members who do) … the process is very simple. They courier the new layer to you and then you replace the layer in your mattress and then ship the old layer back in the same box.
Phoenix