I answered in the other thread, but to summarize: Yes, sail range depend on sailor weight, but other variables are just as important. So it takes two very similar persons sailing similar sails in similar waters in similar ways, bot one being heavier than the other for a comparison to make sense. And different people might even have differnt reasons to change boards size at all. For example, with my wave boards, I more choose boards depending on the wave conditions. Pretty much all my boards go with pretty much all my wave sails. Might choose 70 liter with a 5.5 if waves are good, but might sail an 80 liter with a 4.0 if its B&J conditions.
I think this particular thread clearly shows how much personal preference can shift things around. At least with a bit of experience, you genreally know what kind of size you like. And of you don't, I think it is a much safer bet to just ask someone more experienced, for example on this forum, than try to read some complicated schemata of sails, boards, weights, conditions, skills etc etc.