Hi Joel,
There are some general guidelines about weight and height here and some general guidelines about sleeping positions here and some more information about how different types of layering combinations can affect these guidelines here (and in the other pages in this section) as well.
Overall it is far more effective to base a layering combination on the results of your personal testing though rather than what I call “theory at a distance” (especially without a specific reference point of a mattress you have tested and where the layering is known). There is no weight/height/sleeping position formula that works for any individual except in terms of averages (and none of us are really average) and even averages needs to take into account all the components of a mattress.
Outside of personal local testing on various mattresses where the layering is known … the next best way to go is to work with a manufacturer that sells mattresses online and has the feedback from a large customer base and the skills and experience to help you make the best possible choices with the specific combinations of materials and components that they offer. Even seemingly small differences other than the foam in a mattress can make a real difference in how a mattress feels and performs and the guidance of an “expert” that knows every detail of the mattresses they sell can be invaluable in making your best choices. There is a list of these that are members of this site in post #21 here. There is a wide range of different options and budgets represented there.
The third option (after a local purchase or working with an online manufacturer) can be the most risky of all because it generally takes extensive testing to know exactly what you need and prefer and knowledge of how all the different layers and components of a mattress can interact and affect each other.
Some of my thoughts about the three main ways of buying a mattress (local, online, and buying all the layers separately) are in post #15 here. There is also a list of DIY suppliers of various types in post #4 here.
In general … combination side/stomach sleepers would do best with a firm core and then “just enough” of a comfort layer to provide good pressure relief on their side because the thinner and firmer it is the better it would provide the necessary support for stomach sleeping. You don’t seem to be a full side sleeper so you may do well with a little thinner comfort layer than a typical side sleeper.
Overall though the risks increase as you go from option 1 (local) to option 2 (online) to option 3 (DIY by yourself) and IMO to go with the full DIY route there would need to be a significant difference in cost over option 2 to justify the extra risks involved (and the extra costs if you make mistakes and have no recourse or the recourse involves significant extra cost).
In most areas of the country there is usually either good value available locally (and if you let me know the city you live in I’d be happy to let you know of any better outlets in your area I’m aware of) or alternatively available through manufacturers with great quality and value that specialize in working with their customers online or over the phone and the the benefits of bypassing these options and using option 3 would need to be significant to offset the risk and potential high cost of working “without a net”.
Phoenix