Its rolled on best first, what the item it will be axe or shield or what ever, then if it is unique to rare to magic to normal.
I can't rememeber if it is the type of item first or grade of item, but the rest is correct.

500% MF is the best pay off, your break points after that give very little extra chance.

but ofc if you can get 1000% mf with max res and big damage it would be worth it, but you would maybe only have 10% more chance than me with 500%