Hi foghat,
Post #2 here has more about making comfort or design choices but theory is so complex and ILD is such a small part of a design and there are so many other variables and unknowns that I would either use your own personal testing on a similar mattress as a guideline of if that isn’t possible or practical the best way to choose is more detailed phone conversations with a retailer or manufacturer who knows more about their different mattress designs than anyone else and is in the best position to help you choose the options that have the best odds of being successful for you.
Softness and firmness is very subjective and what feels soft to one person can feel firm to another. Different foam manufacturers also have different methods of measuring ILD so ILD comparisons can be very misleading between different manufacturers or different types of materials where ILD is not necessarily an apples to apples comparison. You can read a little more about this in post #6 here.
Softness and firmness is also relative to which layer you are talking about. If you are talking about a comfort layer then generally 19 - 24 ILD and lower would be considered soft (depending on body type and perceptions or type of material and the method used to determine ILD) but in a support layer then firmer layers would be considered “soft” relative to other support layers. 28 would generally be a “soft” support layer and a medium or for some people even a firm comfort layer.
Again … I would read the information here as you would a good book but not study it as you would a textbook for a course. Connecting with experts is a much simpler and faster process than becoming one yourself because they already know what you would otherwise take a very long time to learn. Designing your own mattress or making choices based on ILD alone or any other types of specs in isolation from all the others can do more to confuse than anything else.
For a support layer 28 ILD would generally be considered to be on the softer side for support layer and this and higher (not lower) is typically used as a support core. Softer than 28 ILD (again depending on how the ILD is measured) would probably be too soft as a support layer for most people and moving into comfort layer territory.
Phoenix