Hi glassact,
As Pist0lpete mentioned … a “bowling ball test” would measure how far you sink into a mattress if one part of your body weighed 12 lbs so it would be one measure of “softness” but it wouldn’t tell you how far parts of your body that weight more or less than 12 lbs sink into the mattress or how soft the mattress would “feel” because of other differences in specs between materials that are used that can also affect softness (see post #2 here) or more specifically pressure relief.
There are also different “types” of softness that some people may be more or less sensitive to (see post #15 here).
So while ILD (which is the equivalent to what you are suggesting) would be one measure of softness and can be a useful guideline … there would still be a significant difference between how soft two different mattresses “feel” to different people even if the ILD of the materials or the “bowling ball test” indicated they were very similar. In the end … each person’s own experience and “definition” of softness is really the only “true” definition of softness that is reliable for any specific person.
Phoenix