Sweet umbrella!
The correct grip keeps the forearms vertical and the wrist straight. This can generally be anywhere from 14-18" depending on anthropometry, with of course some deviation either way. I always put my index finger on the start of the knurling where it meets the center smooth (on a standard barbell with a 16" center part) and that lines me up nicely. You're 12.75" grip is about right, although I might have you play around with a knuckle or two wider than that to get your wrist perfectly straight, as it looks like it's a little radially deviated to me. The 18" grip is too wide.