Just go to look for it and wow
. I can't navigate the new Arnies at all.
Anyway, it was in the download section.
It doesn't do it directly from .12g to .20g, but if you put the .12g velocity in and write down the joules it gives you, you can try until you get (nearly) the same number in .20g.
By the way, according to the calculator, 300FPS with .12g is 232FPS with .20g.