I agree with Krister.
I've also had the F161 and believe me, it does not work with a 50cm fin. I've also tried it once and the board felt like dancing around in circles. It needs 60+ fins. Go for the smaller sail or if you can spare the money choose a board like the Ultrasonic. It's so much easier than a formula, especially when the wind picks up. It would be perfect with your 10.6 sail. I use very often a 10.9 sail on it. Planing in 8-12 knots but easily controllable up to 15 knots as well. Super !
IS87 / IS101 / IS122 / US147