Hi BostonMatt,
There are some topper guidelines in post #8 here that may be helpful.
There are too many unknowns and variables and individual preferences to use formula based on “theory at a distance” to predict your own experience on a specific mattress / topper combination in “real life” and the only way to know with any certainty would be your own personal testing or sleeping experience. The specifics of your mattress will also have an effect on the feel and performance of a topper (the same topper will feel differently on different mattresses).
The key with stomach / side sleeping combinations is “just enough” in terms of thickness and softness to provide good pressure relief on your side so that there is less risk of alignment issues when you are sleeping on your stomach. As you suggested … I would start with 2" and then add more if necessary because 3" may be more than your ideal for a stomach sleeper.
There are also many good options in the Boston area that make or carry latex mattresses and / or toppers that may give you a chance to test various toppers on top of a mattress that you think is fairly similar to yours so you have some good reference points. They are listed in post #2 here.
I would also consider that Dunlop ILD’s are not always accurate (see post #6 here) and I would be cautious in using them as the basis for your choice opr comparisons unless you are confident that the retailer you are dealing with has listed them correctly and the densities seem to roughly correspond to the density (see post #2 here as a reference). ILD ranges with Talalay will tend to be more accurate but ILD will also vary between different types of latex.
I would be very cautious about buying latex from Foam by Mail (see post #2 here and post #2 here) and the topper guidelines include a link to some of the better topper sources I’m aware of.
If you aren’t able to test the same or a very similar mattress / topper combination in person to use as a reference point then a more detailed phone conversation with each supplier you are considering would be your best source of specific information about their toppers or for guidance about which one may work best for you. The exchange or return options may also be an important factor in your choice.
You can also read more about the different types of latex in post #6 here because the type and blend of the latex will also play a significant role in the feel and performance of the latex as well as the price you pay.
Phoenix