OK, if you can't have a 100% as a percentage (I see your point), how about grounding it in real terms?

Protection up to 100 degrees celcius?

Protection up to 25000 volts?

Or staged protection
99% up to 50 degrees
80% up to 90 degrees
40% up to 110 degrees
20% up to 120 degrees
0% thereafter

Then the damage would be realistic... as would the messages

Monster attacks player with 200 degrees C, player does the flambe fandango

Monster attacks player with 50 degrees C, player says, is it me or is it hot in here..

Armour could have a heat dissipation flag over a period of time so that repeated attacks would add heat upon heat.. so 50 degrees and 50 degrees when being attacked by multiple creatures would add the values together.. or if a monster attacked repeatedly one after the other in quick succession, the original heat would not have fully dissipated and the new heat would be added to the remaining heat, doing appropriate damage..