If you double grains or extracts then of course you also need to adjust yeast and water useage as well since the boil off amount will be affected by the increase. Not sure the math on it but I know it isn't difficult to do and varies depending on the dimensions of the pot you are boiling in as well.
Catch the next Triple Barrel Brewcast Broadcasting now 4/1/13 tune in and enjoy
didn't take that into account on a kit double ipa recipe. Split wort in half in two fermenters and the the o.g was way off of the recipe and different per carboy. I'm not sure what is going on.