Hi bear,
There is really no “theory at a distance” that can choose a topper for someone else because it depends on the specifics of the mattress it’s being used on, the body type and sleeping positions of the person, and individual preferences and perceptions. Having said that … post #2 here and the posts it links to have some guidelines that should be helpful that can use how your mattress currently “feels” to you to help guide you.
A good “average” choice would be in the range of 2" and a “soft” talalay topper for most people would be in the range of 19 - 24. Each person can feel a specific ILD differently and for example lower weights would usually feel similar with a lower ILD than higher weights would feel on a higher ILD. This means that I would lean towards the lower end of the “soft” scale because your weight is low. It really depends though on your own “best assessment” of how much more softness you are looking for. Outside of actually testing a topper with a specific mattress (which is really the only way to know with certainty) the guidelines in the linked post along with phone conversations with a knowledgeable manufacturer or retailer that you are considering can give you the chance to go into more specific detail about the layers of your own mattress and what you are looking for and any other information that would be helpful to help them use their knowledge and experience to give you some insights about which topper would have the best odds of being suitable for you.
If you are still too uncertain after all of this then the return policy for your topper may also become an important part of your decision and “personal value equation”. In this case I would also make sure that you know the specific return policy for any purchase you make so you can “balance” the risk of a purchase you can’t test ahead of time with your confidence of success, the cost, and with the options you have after a purchase.
Phoenix