Hi doxn,
I would probably suggest 2". Based on “averages” … the mattress you chose already has a soft comfort layer which would have worked well for most people of your height and weight who preferred a softer surface and if you add too much more of even softer latex you would be increasing the risk of alignment issues … especially on your back because you may be too far away from the support layers of the mattress. 2" “should” be plenty to give you the extra softness/pressure relief you need and it’s not likely that you would need more.
It may also be worth considering the Seven Comforts topper which has latex “rods” instead of a solid layer and may provide some extra softness and pressure relief with a little less risk of alignment issues that comes with thick soft layers where your pelvis/hips may sink in more relative to your shoulders. If you go in this direction and the size you want is not available on Amazon … their main site is here.
Edit: see post #52 here first if you are considering ordering this topper.
Phoenix