I can't say for sure. the formula that I used was taken from what I know about upholstering chairs. I don't claim to be an expert, but with chairs you do layers from very dense to very soft. I have a bunch of foam scraps at work to play with, and did, until I was reasonably happy.
With all of that said, I must confess that after living with it for a while, I think I'm going to re-do it. It really is fine for the amount of riding I do (mostly commute) but it's too hard for much longer then that. I'll start doing some research and post back. Please do the same if you find anything out.