I sail 8.0 and 10.0 Retros on my Free Formula 198 (90 cm x 280) and I weigh 190 lbs. I often check the wind speed after I get home online with NOAA. It seems like I need about 11 - 12 knots to be consistently planing on the 10.0 and maybe 12 - 13 for the 8.0.
Roger give me a good tip a couple of summers ago about planing earlier. He said not to sheet in too much. I would see a gust coming and oversheet which stalled the sail and made it harder to get going. I now try to sheet in lightly, bear off some and pump just a little. I still have days where it seems windy enough but I just can't quite get planing.