Howdy!
Whats a good number to cap at for hell Andy/Meph/Chaos/Baal Runs? I know you can get too high and go overkill. Let me know!
Killspeed > MF
But if you could somehow have 0 mf and a 30 sec kills or 500 mf and 33 sec kills on andy, I'd probably choose the latter.
Realistically, it depends on your class and build. 3 piece tal sorc can get 400+ mf easily. A DPS oriented cold sorc would have 100 mf or so, and be able to clear bosses faster, and clear chaos + baal + TZs MUCH faster. I choose dps over mf here.
Runes and charms are not affected by mf, and higher mf reduces the chance for white bases. If you are hunting bases, you want lower mf.
Bosses always drop magic (or better quality) items, so more MF will improve the chances of rolling a unique/set/rare. If you are target farming act bosses, more mf will be better.