Hi okoebordnerd,
The best way to do this would be by using their mattresses and your own personal testing as a reference point instead of trying to “match” another mattress which may not be possible except subjectively based on your testing and perceptions.
You may not be able to find out the information you need to “duplicate” the Serenity. You would need to know the specifics of every layer in the mattress including the comfort specs and the quality specs and it’s not likely you will be able to find this out. The Eco Comfort has the following layers (from their site) …
Eco Comfort Serenity
High Loft Fiber 2" Hypersoft: You would need to know the density and the ILD of the polyfoam used in the quilting. I’m not so sure that I would even want to duplicate this and use 2" of softer polyfoam over latex.
Comfort Layer 3" W4 Natural Latex: This looks like blended Talalay made by Radium and would be between 17 and 20 ILD.
9" Soy Based Omalon Foam: You would need to know the density and ILD of the polyfoam.
There is more about trying to “match” one mattress to another in post #9 here but I would tend to evaluate all the mattresses you test against a common set of criteria rather than trying to “match” another mattress (see post #46 here) which may not be possible unless both manufacturers use the same design and materials in the same firmness levels in their mattresses.
My best suggestion would be to work closely with Jim who will know more about their mattresses and the options they have available than anyone. They can work with you in person instead of using “theory at a distance” which is never as effective as your own personal testing and perceptions about which of their mattresses is the best “match” for your in terms of PPP.
Phoenix