Reading the info for the Mage Armor perk for Alteration, it says it doubles the effectiveness of armor spells if you are wearing no armor yourself (cloth only). I noticed there are three ranks to this spell, so my question is, how is this calculated?

lets take the oakshield spell as an example, it provides 40 armor when cast. if I have rank 1 of the Mage Armor buff, it should scale to 80 armor, correct? with the 2nd tier of this perk, will using that same spell now improve to 160 armor? and the next tier 320?

If anyone knows it would be greatly appreciated!