I mostly sail on lake Geneva. Home spot area covers about 3-5 km wide x 5-8 km long. It's not uncommon to have different wind speed depending whether I'm near the north shore or the south shore banks on a particular day. For example, I can get 10-15 knots near the north shore and 12-18 knots near the south shore at the same time same day. At my home spot, all wind speed are recorded on-line at www.windspots.com
(Switzerland, lake Geneva, Geneva, La Nautique (=south shore), Le Reposoir & Versoix (= North shore) and www.vengeron.net
(= north shore). Beware of system glitches or break-down (e.g., flat wind speed recording and it's obviously blowing wind as witnessed by the webcam images)...!
The first thing to do is to verify the accuracy of these on-line anemometers. Post yourself near one of these and record the wind speed + time with your own hand-held anemometer. Compare and check for any discrepancy, i.e., wind recorded on a post at 10 m high is usually a tick stronger than at 1.90-2.10 m high when you hold your anemometer with your hand above your head.
Never believe the wind speed of an on-line website anemometer if you didn't check it's location first hand. In particular, check for possible wind shadow. Depending on the wind direction on a particular day and time, some anemometer are in complete wind shadow and consistently under-report the true wind speed on that day, e.g., especially by offshore wind. Trees, building, anything that can block the clean and steady wind stream around the anemometer will give erratic and unconsistent wind speed records.
Finally, in true 5-10 knots windy days, you will be schlogging 100% of the time at 5 knots wind and planing 100% of the time in 10 knots wind for your 68 kg with a 9.5 m2 sail on your Fanatic 75 cm board + 52 cm fin, no question about it.