decrepit said..
I'm 65kg and use a 7.0 on a 90l board, in 8 to 15kts. You already have big boards for the very light days. I wouldn't contemplate using anything bigger than the 7.0, but then most of our wind is closer to 20kts
I need more wind than that to get planing on my 90L board. More like 16 - 18kn. Decrepit is a much better sailor than me, but his board may be a bit different too. Mine likes to be really powered up. It is not an early planing board. Depends on tail width I guess, but that starts getting complicated.