There are two major ways.

1. Take a dull, NON FLARING, ball. throw it and mark the oil line where it was rolling.  Then find the point on the positive side of the ball that is equidistant from the marking line.( Ball doesn't HAVE to be dull, just helps you see the oil ring easier)

2. Take a flaring ball. Throw it and find the FIRST oil ring and mark it. Then follow the steps above


P.S. In the old days, when ball didn't flare, we used to use a piece of white tape on the side of the ball.  We just kept adjusting it and throwing it until the tape seemed to remain stationary on the ball. That was considered to be the p.a.p. This WILL NOT WORK on a ball that flares!!!!!!!!!!!!!!
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HAPPY  NEW  YEAR!