I often begin with an 8x10 print even when I know I will ultimately print it larger. My experience echoes those above: it is more than just doing the math. So, as Brian says, I also use small pieces of the same paper that I intend to make the large print on, and pretty much start over with new test strips. My precious RH Designs timer automatically scales up the dodges and burns that were worked out for the small print, and those are usually pretty close.
I was fortunate enough to spend a day in the darkroom with Les McLean one time. I thought it was kind of crazy when he told me that he pretty much starts from scratch each time he reprints a negative, but I (wisely, I think) kept my counsel. What he said makes more sense now.