A few simple rules can be applied to maximize the accuracy for most probing applications.  

A = ball diameter

B - Overall Length

Renishaw uses a standard description of overall length, measured from the rear mounting face of the stylus to the center of the ball.

C = stem diameter

D - Effective Working Length (EWL)

This is measured from the center of the ball to the point at which the stem will contact the feature when measuring 'normal' to the part. EWL can also be affected by assembly tolerances. For this reason Renishaw styli are assembled to exacting standards in controlled conditions. quote request g2metric