In a nut shell I am looking to display the margin of error from a
perfect fraction. So I can determine if a orbit is is too close to
being in perfect resonance and hence unstable.

0.61518624 = Current real value = 1,922,457 / 3,125,000
0.615384615 = Exactly 8/13 two significant denominator digits.
0.032% <= difference express as a percentage.

Anything in the denominator 100 is statistical irrelevant

how can i get an excel cell to do this automatically?