Only rule of thumb that I can recall right now and I am very sleepy is that you want STR > Encumbrance rating.
I've heard this as well.
I wish I had a really solid quantitative answer for you...but I don't really. My skill training is pretty haphazard and probably quite bad. Pretty much I just train things until they feel about right and I usually train too much at once.
I guess my rule of thumb is that level 10 is like being "pretty good at something", without requiring massive skill investment. So I try to get my core skills like dodging, fighting, evocations, etc. up to 10 on pretty much every character I play. If I'm wearing a robe I usually won't bother with armour skill, but even if I wear mottled (and I love to pick up the faerie dragon armour when I can) I'll try to get it to that point. If I intend to wear plate or something I'll probably just leave it on for most of the game.
So it's hard to answer your question without a real "feel" for the character. For example, if I'm getting a bunch of "Your plate mail prevents you from attacking!" messages then I'll be more likely to focus armour, because that sucks right? Honestly that scale mail looks pretty good to me, I might stick with it for awhile. I'm sure you've got the strength for it. rElec is hard to come by. You're not casting any spells anyway I guess, so wearing some armour won't hurt that, and no translocations makes the evocable blink look double appealing.
And +9 AC is like really really good, especially in the first half of the game I'd say. EV becomes relatively more useful later on. So if you sacrifice some EV now and pump armour skill, you'll notice both your AC and EV rising, and I think you'll eventually hit some kind of armour skill "magic number" where dodging becomes really good again, so you can really capitalize on your aptitude for it. Damned if I know what that magic number is, and it's probably non-trivial to calculate, and I might have just made that up. Play it by ear.
And then of course you can just wear the +15 faerie dragon armour of amazingness when you find it!