Yeah, exactly. That's why I went with something simpler, like X armor blocks X damage, but can only block at most half of an attack's damage. It's simpler, but lets you ramp up the damage reduction numbers such that they might actually work against big guns without giving you total immunity against small guns.
Flat percentages on items aren't great either because you either get big numbers early and they never go up, or you have to give small numbers early and nobody wants 2% armor when they're being shot for 20 damage.