How much would 10/20/30/40/50% in damage reduction equals to in armor points?

There's so much hidden values and crap with Skyrim damage reduction that it's anyone's guess how much is really being blocked.

Each point of armor rating grants 0.12% damage resistance, with the added wrinkle that each piece of armor (including shields) grants a "hidden" 25 points' worth of armor rating. Working backwards, divide damage reduction percentage by 0.12%, then subtract 25 for each point of armor - this will tell you the displayed armor rating that will give that reduction. So, for example, a DR of 50% with full armor without shield (4 pieces) would give a displayed AR of ~317.

Keep in mind that a displayed AR of 567 only gives a DR of 80% if you assume a full set of armor without a shield. With a shield, a displayed AR of 542 would put you at the 80% DR cap.

But, yeah, the way AR itself is calculated is weird. More details http://uesp.net/wiki/Skyrim:Armor.