At some point it gets to 20 and 25% but I do not own item levels in this range presently.
Can you let me know the type of item? I' am working on formulas for the various boosts and I need data. I had a hunch that only Item Level plays a part and your data seems to support.
I think it does not matter which type of item, it will be the same for sarong, belt, gloves, armor or helmet (buckler as well
The armor boost with fire resist modifier = 3 comes to be L1 = 5% L2 to L6 = 10% L7 to L11 = 15%
If the modifier is 7 such as with "Fireman's", "of Fire Resistance" gear then it is: L1 to L3 = 15% L4 to L3 = 20% L9 to L10 = 25%. Presumably this follow the 5 levels step so L13 = 25%, and L14 = 30%
Or so I've observed while playing with the data. Still trying to get the formula