I think I simplified it even more. Seconds/mob, since that's what Sam seems to like. It starts out in RFC and Stocks being pretty high, maybe 4-5 seconds/mob. Maybe less. As you level up, the pulls start shrinking around level 40 or so, but this is made up for by the booster's level having less of an impact on the diminished XP. I would say though if you're ever dropping to 30seconds/mob, you're doing something wrong terribly terribly wrong.