If you're into hip hop, the answer is "can't get there from here"...
IF your booth can do 45 dB STC, and you monitor at 80-85 dB "C" weighted WITHOUT a sub turned on, you should have enough isolation for it not to be a problem.
If you monitor at "show-off" levels WITH a sub, then just move your vocal booth down the block and you'll be fine.
Otherwise, I hope you built your booth with double $6k Overly doors, double 8" concrete walls, 5" thick double glass, and a completely separate HVAC system... Steve
na my booth is good. I get about 40db or so, as you said. I usually listen back at like 40db on my Hafler M5'S(5.25" 2 way) while recording. Works good. I just was curious how other folks where doing. I mix on Mackie's.