ok well if your usbs are rekordbox analysed, that bpm analysis has been done by the computer and not the CDJ anyway, so its usually fairly accurate (as far as BPM analaysis on a computer goes). Its more accurate than using non-rekordbox analysed usbs or CD's. you can hook your CDJS up to your laptop using LAN cables and play tracks directly from rekordbox on the cdjs if you want to use your computer, but the end result is the same as using usbs. the bpm counting is done the exact same way etc. you just have your laptop screen to look at