Hi bmcken1829.
Generally, thickness and firmness are related. Thicker mattresses will “act” softer for most people. If you make changes to one of the specs (such as the layer thickness of the middle transition layer) … then you may also need to make other changes to the other layers to compensate. All in all, adding an extra inch to the middle layer will add more cradle and you’ll sink in a bit more. As you are a stomach sleeper and also lightweight you wouldn’t need too thick of a comfort or transition layer and thinner layers may be the answer. Depending on the firmness choice and thickness of the transition layer in some cases, even 2" of comfort layer may be too much.
There are many interrelated variables and unknowns for you to take into account, and no “theory at a distance” can replace that. Basic height/weight/sleeping position information or any other person’s experience to choose a suitable softness/firmness combination in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences). is not as reliable as your own careful and objective analysis and testing (using the testing guidelines in the The mattress shopping tutorial )
If I understand your question … the " number refers to the topper thickness.
Let us know of your eventual decisions and additional questions as you move forward with your DIY.
Phoenix