2% BA is the usual for antimicrobial properties.
500mg/ml is pretty good, but pretty thick with no solvents. Be sure to keep it nice and warm when you're filtering.
Also, its not that you shouldn't use solvents...just that you dont need to technically. I continue to use BB @ 20% for everything. It just keeps things thin, and I feel it helps to avoid some of the golf ball lumps many of us are familiar with.