Hi gardenguy,
You are correct that there is no way for me to predict what you may or may not like for your new “transition” layer, due the many variances and subjective nature of selecting comfort layers.
I would avoid attempting to get too detailed in using empirical numbers in attempting to predict what a mattress “might” feel like, as unless you are very experienced with mattress design and how different materials interact (which would be very few people), attempting to get too technical only serves to drive you further down the rabbit hole into paralysis analysis. I hesitant to provide too much extra information, as I’m fearful of contributing to this and thus not being helpful to you, but I do understand your desire to learn, and you do have a reference already of some combinations.
With that in mind, here’s how ILDs are measured and how polyfoam and latex ILDs are not directly relatable. In addition to this though, ILD is not the most reliable indicator of how soft or firm a layer will feel. Compression modulus is even more important because very few people actually sink into a layer exactly 25% and compression modulus is the rate at which a foam gets firmer as you compress it more. Latex has a higher compression modulus than polyfoam. Also, latex is very “point elastic” which means that a smaller area can compress with less effect on or resistance from the surrounding area than polyfoam. This is much like the difference between pocket coils that act individually and innersprings that have helicals that join the springs together so that the compression of each spring will affect the springs around it which makes the spring stiffer. Latex also has a lower hysteresis (how much energy is absorbed) and conversely a higher resilience (how much energy it returns) than polyfoam so there are also factors that can make latex feel firmer depending on how much a specific layer is compressed in a mattress. Because of its unique qualities and ability to take on the shape of the person on it (point elasticity) it can feel softer and firmer at the same time and some will feel it as one or the other depending on what they are more sensitive to, their body type, sleeping position, and how they sink into the mattress.
Overall, I would tend to take step back and look at the big picture a bit. You’ve decided you like Talalay over Dunlop. If the 19 ILD Talalay on the polyfoam core felt good but you desired a bit more plushness, then with your size the 24-27 could be a good choice. If you were “feeling through” the plush Talalay to the core, then the firmer Talalay at 28-32 would probably be a good choice, but I don’t think this is what you’re describing (that combination would tend to result in a bit more “mediumish” to “medium-firm” feel).
That’s about as “boiled down” I can phrase it without getting into the weeds too much. I hope that helps you as you go through your decision-making process.
Phoenix