Gshock and MentalQuest hit the nail on the head. Highest MF you can get without lowering your killing speed.
Just another example on why this is true. Consider 2 possibilities. 100% mf and 1000% mf. What is the chance of getting a windforce with these two values?
0% - 0.000010066
100% - 0.000020132
1000% - 0.000039768
These values are from stats.diabloii.net looking for drops from pindleskin. The chance with 1000% is .003% of pindleskin dropping a windforce. The chance with 100% is .002%. 50,000 times .00002 = 1. So, with 1000% mf doing 50,000 runs should give you a good chance at finding a windforce, theoretically. So, the breakdown is like this:
0% - 100,000 runs
100% - 50,000 runs
1000% - 33,333 runs
Now, anyone who knows statistics will tell everyone that my simple math is crap. Why do you care that 50,000 x .00002 = 1? Well, the 1 signifies 100% chance of finding said item with that many runs and that % of mf. We all know, there are no guarantees of finding an item, so that math just gives an idea of how the number of runs relates to different percentages.
Lets assume the chance of a quarter being heads is an equal 50%. If I flip a quarter 4 times and 4 times it turns out heads, then what's the chance if I flip it a 5th time that it will be heads again?
Ok, before I answer that, let me ask you this. Lets assume the chance of a quarter being heads is 50%. If I flip a quarter 5 times, what is the chance of getting heads every time?
So, now for the answers. 50% and 3.125%. Why? Once a quarter has been flipped (or pindle has been killed) that no longer matters. What happened has happened, and has no bearing on what will happen. Now, if you want to know what the chance is of something happening in the future, then you can compute it if you have the right information.
Just for fun. How you find out the chance of a quarter going heads 5 times: the chance of getting heads once is 50%, the chance of getting heads again is the chance of getting heads the first time times the chance of getting heads the second time (.5 * .5). So if you want to know the chance of getting heads 5 times in a row it'll be 0.5 * 0.5 * 0.5 * 0.5 * 0.5 or 0.5^5 which is 0.03125 or 3.125%.
So, what's my conclusion? Get a good and fast killing time. Then get as much MF as you can without sacrificing your killing speed. Why? The chance of getting a good item is PITIFULLY SMALL. 1000% * PITIFULLY SMALL is not that much better then 100% * PITIFULLY SMALL. So, change what you can. Increase your killing speed while keeping MF as high as possible.
The only exception to this is MF under 100%. If your MF is under 100% so you can shave off a second or two average run time, then you should add that second or two to get at least 100% MF. If you can shave off 1-2 seconds to get 800% MF instead of 600% MF, then stick with 600% and do it faster.
Oogk, the rambler.