"How am I doing?"
Translation:Quomodo me habeo?
I've never seen habeo used the way it is in a couple answers like this. I would have expected agere to be used for doing. Is this an idiom, or standard usage?
habere + the reflexive pronouns (me, te, se, etc.) can be used in the sense of "to be well." I agree with you that the quid + agere forms are more common.
If an idiom or a standard usage, when and where was it idiomatic or standard?
I studied Classical Latin at school, and conversational phrases rarely turned up in Virgil! I imagine this might be the type of phrase priests say to each other in the Vatican..?! But is it standard, I have no idea...
'Quid (ego) ago ?' would mean 'What am I doing?'
(The 'ego' is not used, unless you need to stress it. 'I' is already there in the o-ending of the verb.)
It's a reflexive construction. This is a more complicated construction for such early introduction, IMHO. And it's an idiomatic construction that is really not well translated literally.
Se is only used for the third person, as far as I know. Me is the first person reflexive pronoun.