Go test it for yourself by reallocating your stats in int for piety, see how you get an extra cast in Astral fire, an extra fire 2, when you don't have a sch, a better flowing rotation even when using thunder 1, and then think oh wait I'm only sacrificing 6-9 determination which is 3-6 extra damage total per cast, if you are using ss food even less. Also trusting a calculator that ignores movement, cast speed in relation to mob skills, ignores procs, and has competition with another just as good calculator one saying ss is the best stat the other saying determination, which if you look at scientific theory, then that fails on both accounts. Literally you are loosing at most 6 dmg per fire 3 cast by trading the determination for reduced time in umbra ice and increase time in Astral fire.