I take any ingredient from the recipe I'm using and multiply it by 3 (for a three gallon BIAB batch) and divide that number by the number of gallons from the original recipe, usually 5 gallons. For instance, if a recipe calls for 8 lbs of maris otter, I multiply 8x3=24 and then 24/5=4.8lbs of maris otter. Remember to do that for EVERY recipe ingredient, including hops.
When brewing I use about 5.5 gals of water total: I heat 3-3.5 gals for my mash water, and as I'm holding that for an hour, I heat another 2-2.5 gals. of water to 170 and to do a mash out. I know that most people don't do this with biab, but I have two boil kettles and and can afford to do it. I've done about 5 batches, and I love BIAB 3 gallon batches because I can do a greater variety of beers. Using 5.5 gals. of water comes out right around 3 gals after boil.