I wouldn't get a formula board for light winds unless you are willing to pump. You probably can plane in 10-12 knots with a 10m sail and formula board, but you will have to pump to get on plane.
The other key question is - how many more days of sailing will you get if you can plan in 10-12 knots? If it is just a few, I wouldn't bother with a formula board. There are days when I could go out on my F160 and 11.0 sail in 8-12 knots, but I don't do it that often because I sail 70 days a year. When I have been in a dry spell (July, Aug & Sept) I will go out at any sign of a white cap (10 knots) on my formula board. I weigh 78kg.
On a light wind day (4-12 knots), planing only happens about 50% of the time. However, it's a good workout with all the pumping.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.