The calculation is based on a timing chart for a hard coded list of FFT sizes matched to exponent ranges. For numbers outside of the maximum measured FFT size the credit value is just an extrapolated point from the largest FFT size.
http://www.mersenne.ca/credit.php?showsource=1
From a quick glance,the largest FFT size is 33.5M or so and credit is computed linearly for larger exponents than 596M