I'm sure there are some easier ways of doing this, but I enjoy doing my brew calculations by hand! You need to figure out the total gravity "points" of your wort. After you've pulled the bag and rinsed it, take a hydrometer reading. You will most likely need to correct for temperature, most hydrometer's are calibrated to 15C, by 20C they are off by 1 point so at 70C it will be very inaccurate. I recommend cooling a sample as much as possible (I put it in my freezer) and then using a correction factor if it is not 15C. There are several sites online that will correct for you, or give you the correction factor. (http://www.howtobrew.com/appendices/appendixA.html) Once you have a reading on your hydrometer, let's say 1.035, you need the total volume of pre-boil wort. Let's assume 6.5 gallons. Multiply "35" (your hydrometer reading without decimals, called gravity points) by the volume, 6.5 gal to give you 228 total points. The total points of your brew will stay the same until you add the yeast. So by figuring our what your target pre-boil SG is, you can figure out how much your off by. Example, your shooting for a final, post boil volume of 5 gallons of wort at 1.055, or 275 points. By simpley dividing your brews total points by the pre-boil voume (6.5 gal) you know what your pre-boil SG should be (42.3 pts or 1.042). So, lets say your BIAB, pre-boil, comes in at 1.035 rather then 1.042. Multiply 35 gravity points, by the volume, 6.5 gal. to give you 228 total points. Subtract your target points 275, by 228 to give you a difference of 47 points. DME yields 40 pts, per pound, per gallon. About 1.2 pounds (544g) in 6.5 gallons will give you about 47 extra gravity points, helping you meet your target.