Don't get fooled by the numbers displayed in the tooltip. Each armour level reduces damage taken by 10% (i.e. ×0.9). Going from level 1 to 2 is just as effective as going from level 41 to 42. It's not linear, it's exponential, (0.9^x): +1 level is equivalent to 0.9^-1 = +11.1% health +2 levels is equivalent to 0.9^-2 = +23.5% health +3 levels is equivalent to 0.9^-3 = +37.2% health +4 levels is equivalent to 0.9^-4 = +52.4% health +5 levels is equivalent to 0.9^-5 = +69.4% health