Should be about 10". Just depends on how much curve you want behind the fins. More curve, narrower at the points. Less curve wider. The crotch on a 10" tail should be about 3-4 inches. Could be deeper or shallower. Whatever looks good to you. I always did mine a bit shallower because I liked to put a trailer box in about 5" up from the points. I found that trailer fins added a lot of versatility and with the box I could adjust for the conditions.