RE: Second fin for FreeFormula 158
Which Free Formula 158 do you have?
The earlier FF 158 that was based on the GO 160 hull or the later
('05-'06) Free Formula that's the same as the F-Type boards.
I'm surprised that your board came with a 70 cm fin.
That's quite big for either of these boards, good for getting going early in marginal conditions, but pretty much too big for even mildly powered up conditions.
I thought the '05 F-Type 158 came with a Drake R13 Race NR 640.
So, I would suggest a 64 cm fin and perhaps a smaller 58-60 cm fin for your smaller sails.
This should help your top speed and control across the range of use of your FF-158.
Hope this helps,