Exactly my point. It's hard to make recommendations if you don't know the goals of the sailor asking the questions. Free sailing or racing a formula board suggest two very different sail / fin needs.
Most formula sailors are committed racers and have learned to handle large sails in demanding conditions. If you are a formula free sailor, there is little reason to push yourself to be able to handle a 11-12m sail in 15-20 knots of wind.
The planing threshold in light winds (under 10 knots) is pretty hard to estimate. Since whitecaps begin to form at 10 knots, that's a pretty good measuring line, but if there are no whitecaps, then it's a guess for the most part. The consistancy of the wind is just as important. Generally for free sailing on my formula board with an 11.0, I need to see a an occasional whitecap to get me on the water. Otherwise, I don't know if it's blowing 5, 6, 7, 8 or 9 knots. Since our wind is so variable where I live, if I see a few white caps, it is usually blowing between 4 & 14 knots. Planing most of the time, but do slog from time to time.