2% armor pen quints

128 armor would be the break even point for being equal to flat armor pen quints, which means it wouldn't really be good for most of the game since you would be better off with flat armor pen quints against any target with less than 128 armor.

128 * 0.02 = 2.56

Against a 300 armor target a full set of % armor pen quints would grant the same thing as 18 flat armor penetration, as opposed to a full set of flat armor pen quints for 7.68 armor pen.

The difference it provides against a 300 armor tank is only 10.32 armor penetration, which is a decent chunk but it isn't game breaking. I think it would be a good addition to the game. It has clear weaknesses and strengths.