Not sure sail size and weight are linear.Infact pretty sure they aren`t.
In lightish winds;or possibly in a particular given wind they could be.But as wind increases the difference in sail size has got to diminish.
A doubling in windpower quadruples the amount of energy/power available.
In lightish winds I find I need about a metre more than average (ie I`ll have a 7 on ; 75k mates on a 6 but in strong winds (ie 4 metre ish) I can get going with pretty much same size sail .
I`ve pretty much got it sorted for sail sizes/wind. But struiggling a bit on smallest board choice.
Agree with you re Acid 86/K89 though.
Thanks for repy.