Hi BaldColumbian.
Welcome to the Mattress Forum! 
[quote]I was curious to gather opinions on the final potential layer of my mattress build.
My current layers, from top to bottom:
3" Puralux Latex Memory Foam from Arizona Premium,
3" 30 ILD Dunlop Latex from SleepOnLatex[/quote]
The only way to know whether any specific mattress design or combination of layers and components is a good “match” for you in terms of comfort, firmness, and PPP with any certainty will be based on your own careful testing and/or your own personal experience when you sleep on it.
While I can certainly help with “how” to choose … It’s not possible to make specific suggestions or recommendations for either a mattress or combinations of materials or components because the first “rule” of mattress shopping is to always remember that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components or which type of mattress would be the best “match” for you in terms of “comfort”, firmness, or PPP or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress), sleeping positions, health conditions, or “theory at a distance” that can possibly be more reliable than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here).
As you are attracted to the idea of designing and building your own DIY mattress out of separate components that are purchased from one or several different sources then the first place I would start 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 (which you have already started). 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 either use the specs (if they are available) of a mattress that you have tested and confirmed is a good match for you in terms of comfort, firmness, and PPP as a reference point or blueprint and try and “match” every layer and component in your reference mattress as closely as possible or alternatively use a “bottom up” approach (see post #2 here).
First referring you to what I just wrote above, I would make a note that, in general, most people would find a 3" “core” of 30 ILD Dunlop latex to be a bit on the soft side, especially with the Puralux slow-recover latex on top of that. Of course, only your own personal testing will be able to determine that for sure.
While deeper layers are “felt” less than upper layers within a mattress, polyfoam and latex are certainly different types of foam with different properties, qualities and comforts. The latex certainly will last longer than the polyfoam (you don’t mention the polyfoam density which is the more important number to know for durability), especially as it fatigues on a year-to-year basis.
I’m not sure to what layers you’re referencing, but only you can feel what you’ll feel, so this isn’t something I can judge for you.
I touched on this a bit in my previous reply. Also, there is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel” that may be useful as well.
Overall, while knowing the specs that can affect the quality and durability of the layers and components in a mattress is always important … unless you have a great deal of knowledge and experience with different types of mattress materials and components and their specs and different layering combinations and mattress designs and how they combine together and can translate them into your own “real life” experience that can be unique to you (which would generally be a very small percentage of people) … I would tend to avoid getting too complex in attempting to predict how a mattress will feel or perform for you. When you try and choose a mattress based on complex combinations of specs that you may not fully understand or only based on specs for single layers or components that may not be as relevant or meaningful as you believe it is then the most common outcome can often be “information overload” and “paralysis by analysis”. Even the best mattress designers in the industry are often surprised at what a mattress they design “should have felt like” based on the specs when they design it and what it “actually feels like” when they test out their new design. Your careful testing of the product you created will be paramount in determining your level of success at creating a comfortable product.
Good luck with your creation!
Phoenix