You're right that the passive bonus armor is included in the total value displayed and not added after that, even though the infocard displays the bonus under the total. So at level 5 a Rad can have 13 armor (of which 6 would come from three levels of EA), but that surely isn't awesome as 19 armor. My bad for believing that the bonus wasn't included in the normal armor displayed, but I blame the devs for another confusing/gotcha UI element in this game. (Incendiary shells passive bonus damage on the Mazra for example ain't included in the firepower displayed in the infocard.)
This is pretty much what the learning curve in this game is all about... bugs/gotchas like that. It reminds me of one of my first posts here when I discovered that capital ship firepower and graphics thereof are not only unrelated but sometimes inversely correlated: https://forums.sinsofasolarempire.com/427922/page/1/.
Besides my overestimation of Rad's actual armor, my general idea for calculating its survivability is actually correct. Let me break it down for you. LFs do only 17.5% of their inforcard dps to capital ships after their (50%) "bonus" [de]multiplier vs caps and 65% shield mitigation of a typical cap, i.e. one LF doing 10 baseline/inforcard dps does 1.75 dps to a cap (of any kind) at that level of shield mitigation before armor and repairs are factored in.
Now adding the [correct] armor and repair plat to the equation: 13 armor (total) transforms ever point of hull repair per second to 1.65 points. So under plat repair (and ignoring the fairly negligible self-repair in combat) a level 5 Rad [specced for EA] can tank 1.65 * 30 = 49.5 incoming dps after mitigation. Now 49.5/1.75 (from the previous para) is about 28. So I've tried 30 [un-upgraded] Cobalts shooting at a Rad under repairs. The Rad stays starts to gain back hull very slowly after killing a couple of LFs. Recording: http://www40.zippyshare.com/v/21301524/file.html
So my general idea/equations for making these calculations is not wrong, even though my understanding of Rad's armor bonus from EA was wrong initially. Which gets me to the point that I should probably start a comparative cap ship survivability guide... e.g. how many LFs are needed to start chipping away at cap X at level Y under plat repairs. Typically, you'll get the cap to repairs when it's already badly damaged so knowing how many LFs are the breaking point under repairs is info I'd like to have at my fingertips. This info is easy to compute from the cap's armor/hull [and repair plat hp/s] alone, but when you add active self-heal abilities [that also need AM and regeneration thereof and also have cooldowns], e.g. the Kortul, it's much less easy to calculate on a napkin. Sure you can wait until you have 60-80 LFs and then it's a quick/guaranteed kill, but a better informed player can take down the enemy's free/initial cap sooner.