Then, as a check, total the north and south halves. This total
should be 79.30, the same as the retracement distance.

It is best to do it both ways for a check. Subtract P1 from the
retracement distance and calculate P2 separately, using the proportionate
formula comparing the record distance for the north one-half mile
and P2, either way the answer should be the same. His total should
be 79.30, the same as the retracement distance.