Ok. That's simple math. Subtract the smaller yardage from the larger and multiply by the number of balls called for and subtract or add that number to the total yardage called for. That is if the hook and guage of yarn is the same but using a larger hook changes that equation.

I'm afraid there is no way of figuring yardage when using a different hook or varying from the pattern. The number of balls is figured out after the pattern maker has finished making the original afghan. When doing any project especially one as large as an afghan I always buy extra because my tension may not be the same as the pattern maker and I'd hate to run out when I'm almost finished.

If not used there is always a project I can use it for. I've made a few lap quilts for my local nursing home from left over yarn done in many colors and linked together with black borders.