as high as possible as long the mf did not affect your kill speed
This.
A pitzerker will reach 850-900 without affecting kill speed.
Max I got to on my Hdin is 616, it is no issue when playing P1 and does not or almost not affect kill speed, but when we start talking P3/5/7 and Heralds, gotta swap out the MF scs and ring to Pcomb and BK.
If you play P1 most chances are you can trade off some dmg for MF and not see a difference in kill speed.
If you play with other players in the game, you want to focus on kill speed first, and find good gear spots that are both useful/give dmg and MF. Which is why shako has been so popular for example, too many good stats and also has high MF. Would people play Shako had it not MF ? Yes. Is it a nice add to have ? Also yes.
The first MF% is the one that gives the most.
After that, each point you put in has diminishing returns for Rare/Set/Uniques.
Beyond 62 250% MF is useless for uniques as you hit last Unique effective MF BP of 249.
One Souls Ritual will get you at 236 or 237 depending on if you have a wee bit of MF or not.