Your weight play a big role in planing threshold. I am big on formula free sailing in light winds, but I usually don't go out unless white caps are forming (10 knots). Even with some caps, where I sail, there will be holes where I drop off plane pretty often.
I weigh 80kg, sail a F160 on 11.0 Maui Sails TR-3 in the light winds. As far as formula range, you can go out in pretty strong winds with practice. Pro racers can manage 30 knots with 9 m sails. I have raced in 30 knots, but I have gone down to a 6.5 sail just for survival. I frequently sail my 8.4 in 20 knots which is great fun. To handle large sails on a formula board in strong winds takes a lot of practice. The down side of big sails of 9.0 and larger is the cost (sails, masts & booms), but it can be worth it.