Quote
Weird math because he said to subtract the new tach time from the old, and then add the difference to the new.

If I have absorbed the algebra and help correctly, I should have noted:

Tach time listed at replacement (say 5400)
Tach time on the replaced unit (I think it was 4760)
and the Tach time now (I think it was 5300)

5400 + (5300 - 4760) = 5940

Appreciate the correction.

Almost! You said it right in the first line, but transposed the numbers in the formula. It should read:

5300 + (5400 - 4760) = 5940

You did it right! You just didn't type it right!