evo - is anybody there?

[or sha]

hey everybody...
i have an evo 92 2005 for couple of monthes now, and i feel that i'm not getting the most out of it.
the main problem is the "late" planning.it makes me get downwind way too much.
i thought about buying a bigger fin (i currntally have 24+28)...
what's the max fin i can put on?
i'm using my evo on 18+ knots b&j conditions and weighing 86 kg.

thanks.