Diablo 3 MF

Killing a champion mob guarantees a drop of magical properties, this means a minimum blue rarity
A rough break down would be
75% chance for yellow, 22% chance for blues, 3% for legendary / set item drop
Magic find applies 100% to blues, 30% to yellows and 10% to legendary /set items
So if you have 50% magic find:
blues chance x 1.5
yellows chance x 1.15
legendary chance x 1.05

In this case:
22% * 1.5 = 33%
75% * 1.15 = 86.25%
3% * 1.05 = 3.15%

Since it's guaranteed 1 item we have to scale those values down to have them add up to 100%
The total is:
122.4%
We must divide all values by 1.224 to have the original 100% back.
33% / 1.224 = 26.9607...
86.25% / 1.224 = 70.4657...
3.15% / 1.224 = 2.5735...

Total: 100%
As you can see if this is the method used to determine the guaranteed drop from certain mobs, chests and events,
legendary chance is decreased.