Hi scubamom,
This is a fairly common comment about latex which has a combination of properties which are unique to latex that make it both soft and supportive. I agree it’s difficult to explain (it’s like trying to explain the taste of a food to someone that hasn’t tried it).
This can be so subjective and vary so much between different people, body types, and sleeping positions that there really isn’t a formula that you can use to accurately predict whether a mattress/topper combination will be a good “match” for someone until they’ve tried it in person. Outside of dragging your topper into the showroom and using it to test the combinations in person (or using one of their toppers if they still sell the same one as you have) … the most reliable way to know would be to choose a mattress firmness level based on your testing and “best judgement” taking into account the difference the topper made with the firmness of your mattress and then sleeping on the combination to assess whether it needs any further “fine tuning”. If it turns out that you need to make any additional adjustments then you can use your actual experience on the mattress/topper along with the information in post #2 here and the topper guidelines it links to to help you decide on any additional materials or toppers you may need to add to your mattress. You would still have the option for example of adding an additional inch of latex to the topper you have (and wrapping them in a single cover) and you would end up with the rough equivalent of a 3" topper without having to buy the full 3".
The biggest “risk” would be to choose a mattress that is too soft because you can always add additional layers or toppers to add additional softness and pressure relief to a mattress that is too firm but there isn’t really an effective way to make a mattress that is too soft any firmer without removing and replacing some of the layers inside it.
Phoenix