In other words, the expression you gave is a close approximation -
within one percent.

Wind, current, and tracking error will offset the acutal results even more, but
99% accuracy is not bad for a memory crutch.

Thanks! (And to Jax who also affirmed the theory)

I have to admit my head started spinning about the second line of your
numerical explanation. (I'm a writer who does words, my wife is a banker who
does math...) But it looks like your process was sound and I've now learned a
handy rule of thumb.