Yep, that is pretty much it.
You can calculate by figuring out how much gravity you are adding
DME is 42 pppg if I remember correctly( pppg=points per pound per gallon)
So- 1 lb DME in 1 gallon water = 1.042
So to go from 63 to 75 in a five gallon batch you will need 12 points times 5 gallons, or 60 total pppg. So you would need approx 1.5 lb dme to make that jump in 5 gallons.