Ok, a quick example.

Say that the oncoming rpm is measured at 26,000 and the going away is 16,000. So one might think that the actual rpm would be the average of the two or (26,000 + 16,0000)/2 = 21,000. But this is not correct.

The actual rpm by the formula would be 19,810.

Another formula is 2x((R1xR2)/(R1+R2)) with R1 the approach rpm, and R2 the away rpm.