Hi djgoldb,
If you are attracted to the idea of designing and building your own mattress out of separate components and a separate cover then the first place to start (and hopefully you’ve read this already since you’ve already purchased your layers) is by reading option 3 in post #15 here and the posts it links to (and option #1 and #2 as well) so that you have more realistic expectations and that you are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process. While it can certainly be a rewarding project … the best approach to a DIY mattress is a “spirit of adventure” where what you learn and the satisfaction that comes from the process itself is more important than any cost savings you may realize (which may or may not happen).
For those who decide to take on the challenge then I would normally suggest either using the specs (if they are available) of a mattress that you have tested and confirmed is a good match for you in terms of PPP as a reference point (the same type and blend of latex in the same thickness and firmness levels and a very similar cover which can also make a significant difference to the feel and performance of a mattress) or use a “bottom up” approach (see post #2 here).
Having said that … since you have already purchase all your layers I would suggest putting everything together (including the cover) and sleeping on it for a few weeks so that the layers have a chance to break in a little and you have the chance to adjust to a new sleeping surface (see post #3 here).
During the first few weeks you sleep on your first combination of layers I would avoid speculating too much about any changes you may think you need and I would also avoid the temptation to use your more subjective sensations (such as guessing how far various parts of your body may be sinking in relative to the others or what your combination of layers “feels like” initially) to assess your success. Once you have spent enough time on your mattress to identify any consistent patterns in your sleeping experience I would rely on any actual symptoms you experience (such as lower back pain or pressure points) to decide on whether your mattress is a good “match” for you in terms of PPP and use your “symptoms” as a guideline and reference point to help you decide on whether you need to make any changes and if you do on the types of changes that would have the best chance of success.
IMO … you are too early in the process to really think about or decide on making any changes yet and I would take a more slow and incremental “symptoms based” approach.
Phoenix
PS: It may also be worth reading post #7 here for more about the “process” involved.