Hi WWB,
There is no formula or theory that can decide on the “right” comfort layer for any specific person (see mattress firmness/comfort levels in post #2 here). The only way to know is your own personal experience with either testing a mattress or “best judgement” based on more detailed conversations with an online retailer or manufacturer who can use their knowledge and experience with their mattresses, any local testing you have done, and the “averages” of their customers that they think may be similar to you to help you make the best possible choice.
Their mattresses don’t have edge support and and for most people a suitable latex mattress doesn’t need it (see post #33 here) but for those who are more sensitive to the feeling of roll off (or “roll together” in the middle of the mattress especially with heavier weights or bigger weight differences between a couple) then in general terms firmer layers in the top 3" to 6" of the mattress (depending on weight, body size, and sleeping positions) will be more supportive in all areas of the mattress including the edge and the middle. If this is one of the more important criteria in your personal value equation though then I would make sure you include it in your more detailed conversations with them because they will know more about the more detailed specifics of their mattresses and how they interact with different people and body types than anyone.
This thread may be helpful 
Phoenix