An easy way to get the exact number that Cromagnum suggested is to make a mark on your front tire where it meets the floor and another corresponding mark on the floor. Roll the bike to make one full revolution of the wheel and mark the floor to meet the mark on the tire. Now measure the distance between the two floor marks, convert to milimeters, and that's the number you use to program your speedo. When rolling the bike it must roll in a straight line. No curve.
Tom