RE: Earliest planing with 8.5 m2
I rode an Ftype 158 with an 8.5 and it was the earliest I ever got going. A local guy routinely uses an 8.5 Retro with his 158 as well. I'd guess that board would be the best in combination with your 8.5 for earliest planing, and as "guest" said, be a better fit than a formula.
I now have a 2001 F155 woodie which is almost as early planing, and just as much fun.