My comment was regarding the metric system in general, not just windsurfing. I use knots and MPH interchangeably for windsurfing, and I think we all use whatever is most common where we live. Nothing is right or wrong, just what works for you.
Experience teaches us how to read the water for wind speed, and it's pretty easy if you only sail in a few locations. It's all about fetch, wavelets, white caps and waves that helps us know what to rig. The hard part is anticipating what the wind will do an hour later. About 80% of the time, I make the right call regarding sail and board, but not always. I hate to re-rig, but it's better than slogging or getting blown away.
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.