> what it's correcting for is an inaccurate value of "c" [...]

I'll leave the logic as-is and just add a note about what is being corrected.

> Numerically, it's probably not helpful.

To make a difference, the mean would have to have huge magnitude relative to the variance; otherwise, squaring the error would drown it out to zero.

The mean() should already be accurate to within a 1/2 ulp.  The summation and division are exact.  There is only a single rounding when the result converts from Fraction to a float or decimal.