Hi Lex,
Im about your weight maybe a little more and I find that the sweet spot with 8m and the sweet spot with the 10m is far enough apart that it makes it worth while having both.
If the winds up and there are white caps im generally on the 10 and if its really windy im on the 8. I never use a wind mtr so im not going to quote knts.
The 8m can handle most winds we get in OZ,unless of cause your wanting to unhhook and do the same tricks that you do in lighter winds in 30knts then you need the 6m.
I only ever use my 12 if im standing on the beach scratching my head considering if its even worth while going out.Wich can be a lot of the time!
I could not live without out all 3 here on the sunnycoast.BUT... if I had to for my weight I would go 8 and 10.
I could not stand missing out on those super windy day's when the ocean is really turning it on

its sessions on those days that stick in your mind for ages and you do not want to miss.
My 2 cents
Regards
Luke