So a while ago I got curious about how the mitigation percentage is calculated, and quickly figured it was linear until 50 %.
Pre 50 % is simply your armor/protection divided by a certain number. 219.6 for armor and 73.7 for protection (at level 80). This number is proportional to your level, which is important later on.
I want to thank cynara for coming up with the method to determining these exact values. I was actually inspired by his blogpost. I wanted to make a long post like his explaining how I went about to find the correct formula post 50 %. But I'm not gonna do that, so I'll just post my finds here.
Well anyway for the formula:
Let a be the same divisor as the pre-50 formula. Lets say 73.7 for protection at level 80.
Let x be the amount of protection you have.
mitigation = 50 ( 2 - 100/(50 + x/a) )
I had some failed attmepts before finding this, but the thing that helped me the most was finding some points in the extremes.
Basically the 1650 protection buff given to a level 2 is equivalent to 66000 protection on a level 80, giving 94.7 % mitigation.
Its the same formula for armor just different values of a.