I apologize for not answering your question sooner, but I'm on my annual tour of Texas and have been teaching and sailing pretty much all day every day.
We just completed a very successful "A Taste of Windsurfing" event at Windy Point, on Lake Travis, in Austin, TX.
60 new windsurfers in 2 days, and $1400 raised for a local charity.
Anyway, I've thought over your question alot, and I don't have a really solid "solution" for you.
Perhaps get a windmeter of some sort and then begin to learn what the differences (the fudge factors) are for the various places you are sailing (the wind measured on short vs the wind you will get out where you are sailing.
This could get you a little closer to using a size that will have you powered up.
Also, check the local airport's aviation weather reports to see what the wind might be blowing and factor these forcasts into your sail selection.
What you really need is to look over the Beaufort Wind charts and see what the different windspeeds (Force 1-6 Bft) look like on the water and on land.
If you see all the signs that the wind is F4 Bft, then you will know that the wind is around 11-16 knots and you will need your 7.0 m2 rig (something larger would be better).
Here's a link to a very basic Beaufort Wind Speed Chart: http://www.spc.noaa.gov/faq/tornado/beaufort.html
Here's a better link with photos of what the surface of the ocean will look like (far out to sea in a ship) at various Bft force numbers:
Overall, you just need to rig at least one sail size larger and learn to tune your rigs for the conditions.
Hope this helps,