Hi mark_in_sf,
Unfortunately there are far too many unknowns, variables, and personal preferences involved for me to make any specific firmness suggestions and some trial and error will be necessary to find the configuration that works best for you because different people can be very different from each other even if they have a similar body type.
The first thing I would decide though is whether you would prefer to use latex as the top layer in your mattress (which would give you more of the “feel” of latex) or whether you would prefer it in the transition layer (which would have more of the “feel” of the top polyfoam layer but would be “modified” by the latex underneath it) because this will affect your firmness choice.
The best suggestion I can provide would be to make sure that the latex topper you purchase has a good return or exchange option so that you can use your own personal experience to decide whether you made the most suitable choice and if you choose a firmness level that isn’t ideal for you in combination with your other layering options that you can exchange it for a different firmness level.
This is what is called a “dominating layer” where a softer layer is used under a firmer layer. You can see some comments about this type of design in post #33 here and the posts it links to and if this is a “feel” that you like then it may be worthwhile using a medium latex layer (in the range of 28 - 32 ILD or so) as a starting point which you can try either over the same bottom layers or use as a transition layer under the soft or medium polyfoam.
Phoenix