Stand straight. Measure your inseam (in cm) from the inside of your foot to the bone @ your crotch. Multiply this by 1.09. Subtract the crank length (usually 17 or 17.5cm). This is the approximate saddle height from the center of the crank to the top of the saddle, +/-0.8cm. I prefer to set mine about 0.5cm below the calculated value. Course the ideal height is also dependent on the shoes and type of saddle.