Mark N7EKU/VE3 has found the calculator on toroids.info to be very accurate — so much that he rarely measure after winding anymore. Toroids.info quotes a quite generous length of wire to use (even considering the 1″ leads) so he usually subtracts an inch.
No need to re-wind if you loose count. As long as you have a cell phone or other digital camera, just take a picture and then zoom in on it. It becomes very easy to count if you do that.
How to wind: first put the wire through the center, gather the ends, and pull them taut. Now wind one half of the windings, and then the other. This way you only have to deal with one half the length of wire at a time (instead of threading the entire length through for the beginning turns).