I'd say "as much as you can carry without noticably lowering you kill speed". It's not easy to get over 580 in an efficient runner, and even if you do, the insignificant drop in set drop chances is more than compensated for by the increase in the likelyhood of getting spiffy uniques. But statistically, what you say is true.
I was running Pindleskin with about 450 mf and I found a Templar's Might, but I still havent found an IKSC. I wouldn't worry about the insignificant differences to be honest, just keep running him with your most efficient gear (mf vs time) and eventually he will drop one.
I just go with the old adage "If you are looking for it, it will never drop". Other wise go with what Llathias said. I've mostly heard that having 250 or more is usually a good place to start. Usually give you a decent enough amount to be getting higher drops and still not slow down your killing speed.