Hi jaminjames,
Just for clarification … the latex in the Casper is synthetic continuous pour latex made by Mountaintop foam which will have a different “feel” from other types and blends of latex.
Trying to design your own mattress based on the subjective feel or firmness/softness of another mattress may be more complex than you suspect … especially when the same type of materials may not be available to you or if you are making more than minor changes to a mattress. There is more about the different ways that one mattress can “approximate” or be different from another one in post #9 here.
Softness/firmness can be very subjective so a mattress that feels softer to one person may feel firmer to someone else depending on the design of the mattress and on how differences in body types, sleeping positions, preferences and sensitivities affects the firmness or feel of a mattress relative to different people. In the case of memory foam the temperature sensitivity or response time of the memory foam can also have an effect and foam density isn’t a reliable way to compare different types of memory foam (there is more about the different properties of different types of memory foam in post #9 here and post #8 here). You could achieve a softer feel in many different ways including using a softer support core, using a softer latex layer, using a different type of memory foam that is (or at least “feels”) softer, using a thicker latex layer, using a thicker memory foam layer, or a combination of several variations … and all of these will have a different “feel” even though they may all be softer.
There is also more about the different types of softness/firmness that different people may be more or less sensitive to in post #15 here that may also affect your design and there is more about the different specs that can affect how soft or firm a mattress or an individual layers feels in post #4 here.
Needless to say … trying to use specs (or only some of the specs that will make a difference) to predict whether one mattress will feel softer or firmer to you or even “how” it may feel softer or firmer can be very complex and lead to some surprises when you actually sleep on them in “real life” because the only reliable way to know whether one mattress feels softer or firmer than another one based on your own unique perceptions would be based on your own personal testing or experience. Even the most knowledgeable mattress designers that have years of experience with many different combinations of materials and different designs will often be surprised at the difference between how a mattress was “supposed” to feel based on specs and how it actually feels when they test it in “real life”.
The “bottom line” is that only you can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved for anyone to be able to predict what any mattress or combination of materials will feel like for someone else based on specs (either yours or a mattress) or “theory at a distance” (see mattress firmness/comfort levels in post #2 here) unless you have a specific reference point where you know the specifics of all the layers and components in a mattress and are making very small and incremental changes.
Post #2 here also has more about the different ways to choose a mattress (online or local) and how to minimize the risk involved with each of them if you aren’t confident that any mattress will be a good match for you in terms of PPP.
In most cases … if you are building a custom mattress it will be much more effective to use a reference point that the manufacturer is more familiar with as your starting point and then make smaller custom changes from there.
Phoenix