As I see it, when it comes to "should I attack this city" the only really significant question is "do I have enough siege to bring most if not all defenders into the red zone?"
And even this number could probably be hard-coded. Based on my settings, this number starts around 8-10 and slowly increases by era. (I'm sure different game settings change this around a bit).
By this I mean that it seems like having more Trebuchets (say) means all defenders are red-lined before the 9th (or 11th) Treb gets to step up to the batting line.
Conversely, having fewer (say only four), is wasteful, since your city invading units will have to fight serious opposition. This means you need more non-siege, and that you will have to build any other kind of reinforcement than siege. Both wasteful.
Again, in other scenarios you might need twenty or fifty siege because you have cities with a hundred defenders.
My point is that for a quick odds computation, look no further than the number of my active full-health siege. Or, possibly, a comparison of (number of enemy defenders) vs (number of my siege units).
Obviously with the pre-Construction era as a significant exception!
