The rule of thumb I was taught was one club for every ten miles per hour of wind into you. As for knowing the wind speed, itís just practice. For me, if itís feeling windy, itís probably 10mph. If Iím raising my voice to talk to others, itís about 20, if flags are ironed out flat, pushing 30, etc.
At my level of golfing skill, thereís no point in getting much more granular than that, and I donít know how anyone is accurate to 1 mph without an anomometer.