5dmk2 with 580EX issue

I sometimes use a Stofen, and sometimes bounce the flash. I find I inevitably need a couple of test exposures to dial-in the right amount of flash compensation -- sometimes as high as +3, often more in the range of +1.

I sometimes shoot in M mode and sometimes in A mode. I have not yet found a pattern to my settings and the degree of flash compensation I need.

Thanks for any advice.