No. It depends on what that first 100 means.
If that 100 is 100 armor def and you armor is all +0 then yeah it will be 1 damage or even misses.
If that 100 is armor def and the + in front of the armor (like +8 chain mail) then you have to use the following:
base armor defense + (2/3 * overup defense)
Ex: Chain mail gives 8 def at +0, to find out how many def it gives at +8 you do:
8 base armor defense + (2/3 * +8 overup defense)
8 + (2/3 * 8 ) = 13.3 defense
Do that for all your armor and if that = 100 then you'll take 1 damge and even miss.
The second 100 means damage you take - 100.