According to Hornady's ballistics (drop) charts, a .357 Magnum shooting a 158 jacketed bullet at 1200 f/s will have a mid range rise of 3.1 inches at 50 yards if sighted in at 100 yards.
Shooting a 125 grain jacketed bullet at 1350 f/s, the mid range rise is 2.7 inches at 50, based on a zero of 100 yards.
So, what distance? What difference does it make?
I'd zero the pistol for 100 yards and leave it alone. Any intermediate distance will be no more than three inches (okay, 3.1 inches) from line of sight. Most of us cannot hold that tightly anyway.
If the idea of sighting in at 100 yards is overwhelming, sight it in 'just a little high' at 25 yards and it will be close enough.
Shooting from a rest is not a bad way to get a rough zero, but when you are shooting from a two handed, standing up position, the recoil and geometry of your arms and wrist will make a difference in your point of impact.
Verify your zero with some standing shots. Don't worry about how quick you are, just get good, solid holds and sight pictures and proper trigger let offs.
If 25 yards is too far, start at 5, or what minimum distance the range allows you. You'll work out to further ranges as time goes on.