Hi Dan1979,
I think dn covered everything at least as well as I could and the only thing I could add is post #2 here which has a list of some of the density/ILD ratings for Latex Green 100% natural Dunlop as a reference.
There is also a pressure converter here which can help you convert one type of pressure measurement to another. I would also keep in mind that firmness/softness ratings can also vary by the type of units they use, the percentage of compression they use for testing (some use 40% compression and some use 25% compression), and the thickness of the layer that is being tested (the two most common are 4" for polyfoam and 6" for latex) so “apples to apples” comparisons can be difficult.
Phoenix