shallow water fin
i am 43 and have an old formula 155. i use a 58 cm fin and a 45cm one as well, Yup the 45cm works with an old 8.5. Its not he best performance to get planing , BUT its shallow water friendly, and fast off the wind beam to broad reach when a biiger fin just is too powered up. Still points..OK... there is some spin out but its a tradeoff.
the nice thing about the formula, is that being so wide,( and this is an older board not as wide as the latest pickings) when the wind does die its just so balance friendly getting back to the launch. Easy on the knees, and legs and that what i need at the end of a session then perhaps its gone light and shifty and my energy level is down.
Even in an off shore light wind in the lee of lots of trees i can get the board within 60 feet of shore, then jump off and push in.
so stick with a wiber board and try a much shorter fin, i'd say.