Hi PLH123,
[quote]I am seriously considering the DIY Talalay blend route with 3 3" layers. I would like to achieve a medium firmness to start.
Option 1:
Bottom Support layer: 3 inches of 36 ILD Talalay latex.
2nd layer: 3 inches of 36 ILD Talalay latex.
3rd layer: 3 inches of 28 ILD Talalay latex.
Option 2:
Bottom layer: 36 ILD
2nd layer: 28 ILD
3rd layer: 28 ILD
Which option would be more common to achieve what most manufacturers consider a medium level of firmness?[/quote]
This is the part where you will need to either do some personal testing to find a mattress that “feels like” a medium to you and then use it as a reference point or blueprint for your mattress or alternatively to do some experimentation to find a combination of layers that feels “medium” to you.
Unfortunately here is no “standard” definition or consensus of opinion for firmness ratings and different manufacturers can rate their mattresses very differently than others. Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that is firm for one can feel like “medium” for someone else or even “soft” for someone else depending on their body type, sleeping style, physiology, what they are used to, and their individual sensitivity and perceptions. This is all relative and is as much an art as a science. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress (see post #15 here). In the end … this can be very subjective and how firm a mattress feels to you will be much more important than how a manufacturer “rates” their mattress or someone else’s perceptions or personal firmness rating.
Having said that … I think your first option would probably be closer to what most people would call “firm” and your second option would probably be closer to what most people would call medium although some could also call it “too soft” (especially if they were heavier) and others could call it “too firm” (especially if they were lighter).
I would take one of the two approaches I mentioned earlier which is either testing a local latex mattresses (if some latex mattresses are available locally that you can find out the type, thickness, and firmness of the layers) or use the “bottom up” approach I mentioned earlier where you start with about 6" of firmer latex (either one or two layers) and then use your experience on this to decide on the next step.
If you are confident that either of your two configurations won’t be too soft for you then it would probably be “safe” to start off with the 9" of latex in either one of them and then use your experience on the combination to decide what layers (if any) you would need to add to them.
Again … the return or exchange policies of the sources you are dealing with would also play a significant role in the “risk” involved in any of your choices.
That’s good to hear … and thanks for the update and for sharing their comments 
Phoenix