When an item is dropped it goes through a selection process before it is actually generated. Like Runes for example, while not impacted by Magic Find, start with a roll for ZOD and if it fails, start a roll for CHAM, and so on until one of them doesn’t fail.
Items always have a chance of passing any the check rolls (base drop) but MF increases the chance of making each of the checks.
The effective MF percentages for each item quality are:
-------Effective MF %
|MF% | Unique | Set | Rare |Magic|
|10| 9.62| 9.80|9.84|10.00
|100| 71.43|83.33|85.71|100.00
|200| 111.11| 142.86|187.50|200.00
|300| 136.36| 187.50| 200.00|300.00
|500 |166.67|250.00| 272.73| 500.00
|1000 |200.00| 333.33| 375.00| 1000.00
So your 400% MF is basically a 400% increased chance to see blues and only a 130% increased chance to see uniques (added to their base drop).
Where your farming also has an impact in addition to this and the number of players.
chance for shak of hell meph @ 1 player
1:874 @ 200
1:728 @ 400
1:667 @ 600
vs 8 player at 400
1:594
So if you have a low roll of like 80 and you needed 90+ for a unique then getting 100% MF would have given you a unique alternatively in that same situation and then you get a chance at what item that actually is on a complex roll since I’m assuming at this point the item is decided first and then quality.
And the thing with cows is that they are cleared for white items because that is what their largest loot pool is. When you cow with 1000% MF you are just converting the massive white pool that you normally farm for base items with am equally massive blue item pool. Since MF works against you when searching for base items.
It wont just convert all the white items into uniques. Here is an example at 8 players with 400 MF.
Astreon’s Iron Ward The Secret Cow Level 1:16582820
Harlequin Crest The Secret Cow Level 1:280986