Hi alasearwax,
There are basically only two ways to decide on the firmness of each layer that will work best for you.
The first is your own careful testing on different combinations of layers (hopefully using the testing guidelines in the tutorial).
If you can’t test a mattress in person then the most reliable source of guidance is always a more detailed phone conversation with a knowledgeable and experienced retailer or manufacturer that has your best interests at heart and who can help “talk you through” the specifics of their mattresses and the properties and “feel” of the materials they are using (fast or slow response, resilience, firmness etc) and the options they have available that may be the best “match” for you based on the information you provide them, any local testing you have done or mattresses you have slept on and liked or other mattresses you are considering that they are familiar with, and the “averages” of other customers that are similar to you. They will know more about “matching” their specific mattress designs and firmness levels to different body types, sleeping positions, and preferences (or to other mattresses that they are familiar with) than anyone else.
Since you are also in very different weight ranges and may have different needs and preferences in a mattress … it may also be worth considering a split firmness mattress that has a different combination of layers on each side of the mattress (see post #2 here).
Phoenix