Hi dzignr_tastz,
Even IFD isn’t a reliable indicator as to how soft or firm a material will feel for a specific person because in most cases it only measures softness and firmness when a 4" (in the case of polyfoam) or 6" (in the case of latex) is compressed by 25% of its thickness (and even this varies in different areas around the world and isn’t always standardized) and different materials will also get firmer at different rates as you compress them more deeply (compression modulus). ILD is also not a reliable way to compare firmness between different types of materials (see post #6 here). There are also different “types” of softness that different people are more or less sensitive to (see post #15 here) not to mention that body weight, sleeping positions, and individual sensitivities will also affect the sensation of softness. The most reliable way to tell whether any individual mattress feels soft or firm for any specific person and “how” it feels soft or firm would be their personal experience.
Phoenix