I do have a rule of thumb for silver age keys, but not nec GA...suprisingly, on most all titles, if you pay attention, you can establish data points from which you can make a reasonable value assertion...applies even to the rarest centaurs... that said, there is no subsitute for experience and time (no way to speed it up or put in a simple formula)...
I do have a restored correlation scale I typically use, but not unrestored... OSPG's exponential value increase across grades is a good starting point, but by no means is it even remotely accurate many times...
really would depend on the title, issue, condition, etc