Zero-Phase Glare, how to calculate?

[FordPrefect]

Hello everybody,

I am looking for a photometric formula/function that describes the zero-phase glare, which can be seen e.g. very prominently on the Moon/lunar surface? All I could find on the web so far, dominantly only show graphs, but not formulas. It is needed to program the effect in a realtime simulation.
Thank you in advance for any pointers or help here!

Rafael