Hi sleepwalken,
While I suspect you may have already seen them ... there are some
guidelines here
about weight/height/body shape and some
guidelines here
on different sleeping positions that can help with "averages" in terms of layering. There is also
some information here
(in the overview and the more detailed pages that are connected to it) about how putting different layers together can affect the feel and performance of a mattress. As you can see there is more than just layer ILD and thickness because every layer of a mattress will affect the layers above and below it. For example ... a 3" comfort layer of a certain ILD with a firm support layer will be "somewhat" similar to a 2" comfort layer that uses the same ILD with a softer support layer. While they will not be the same and there are many other factors that give a mattress its pressure relieving and support qualities besides just the foam that is used ... the upper part of the support layer will "help" the pressure relief more with comfort layers that are thinner. Layer thickness, ILD, ticking/quilting, the type of latex and other variables will all affect how a mattress feels and performs. Even more important there are an almost infinite amount of smaller differences in body shape and sleeping positions that can affect how different people with similar heights and weights can interact differently with a mattress. While most people fall into an "average" range and analyzing different "specs" down to the small degrees will usually do more to confuse than help ... there are also many who are outside of any "average".
While "theory at a distance" can be helpful as a good starting point ... personal testing at local outlets that use known materials and the advice and guidance of a knowledgeable manufacturer or outlet that has worked with many customers and has feedback about the specific materials and combinations that they use are much more accurate than general guidelines that don't take into account every component and material difference in a mattress. Some seemingly small differences can sometimes make a bigger difference than many people suspect.
Since you are more looking for averages though ... it would probably be most helpful to do some phone research and ask the outlets that you are considering which of their specific mattresses they believe would best suit your purposes (suitable for a range of guests you may have). Each may suggest something slightly different based on the specifics of the mattresses they make but you will certainly get an idea of the general range you would be looking at (and as you mention it would likely be in the 6" + 2" or in the 6" + 3" range for a "typical" latex mattress).
If you do a search on the forum you will see some feedback about most of the manufacturers that are listed as online sources in the post you mentioned and they make a wide range of mattresses that have different combinations of features, layer exchanges, materials, and prices. While there are dozens or probably closer to hundreds of online outlets and some that offer good quality and value beside those that are listed ... I have had good conversations with all of these and I believe they are among the "best of the best" (which is why I invited them to become members here). Including many more possibilities would likely lead to diminishing returns in terms of research time IMO.
In addition, I intend to build my own bed frame/foundation using pine wood. I seem to recall a certain material not conducive for latex mattresses regarding slats, but fail to remember what it was. Maybe needed to be prepped or something (sorry, totally blanking right now).
I'm not sure what you might be referring to but I think the idea of building your own foundation is a great idea. I would make sure that the slats are no more than about 3" apart and would make them closer yet If I was building my own (although I'm certainly no carpenter or expert on wood materials). I would also tend towards good quality hardwood (or "strong wood") for the slats rather than materials that are glued or laminated and make sure that there was good center support down to the floor to prevent any sagging over time. The goal is to make sure that there is enough surface area to provide good support for the mattress but also to make sure that it has airflow. I would also tend towards
kiln dried lumber
to prevent warping over time and would also use a good quality material on top to help protect the mattress above it. Even stronger would be something
like this
which has two center supports.
If you meant something other than this ... just post it when you remember
Phoenix