250 armor. Unless you're in a progression fight where that 20 fire resist takes you up to the next tier of guaranteed resistance.
If that 20 resist doesn't get you from one tier to another, raid buffed, it's not going to be any help unless a CD like Aura Mastery pushes it past, and then only for that time that Aura Mastery is active.
So check your raid-buffed fire resistance. If it's not with 20 of the next tier (I don't know if the levels are player levels or against mob levels--- hopefully I'll learn something here) - then it's pretty much a wasted enchant except for maybe a CD like Aura Mastery.