Craig-
I've noticed this type of error in the past, too, and I'm curious to
see the responses that come from the Excel MVPs.

In the meantime, can I suggest a minor change to your formula: the way
it's currently set, it rounds the modulus to 4 decimal places, and I
think your intent was to round C3 to 4 decimal places:
=IF(MOD(ROUND(C3,4),0.0005)0,TRUE,FALSE)