I've struggled with this for a while now.

What I have right now is both mathematical and seat-of-the-pants method combined. When I go from 8x10 to 11x14 to 16x20, simply doubling exposure each step will get me close. Then, I'll often have to bump up contrast by 1/4 to 1/2 grade each time to get each print to look similar. While contrast shouldn't change when the size change, how we perceive the contrast to be change depending on size. When I print larger, I print larger because I want the print to look larger. I don't necessary back up farther from the print to look at larger print looking the same size as before. With that in mind, I usually tweak contrast as well. I also need to change my dodge and burn schedule if I'm going larger by say 2 stops worth. Large patch of dark shadow look far more oppressive at 16x20 than did so at 8x10.

So I basically gave up on math to give me the answers. It will get me close and sometimes close enough. Then my gut and eye tells me what to change and by how much.