I just pressed the "abandon level" button in all 5 of my simultaneous levels.
Indeed the AP offered to me all equals to 2/3 x base AP x % of territories taken.
Does that prove anything? Or to be more precise, how should we know, which territories you had claimed at that point? To be fair, I don't doubt your statement and that formula seems possible as well, I'm just saying that you could only know whether this holds true, if you started a level and only claimed the most expensive territories on your way to say 50% of all territories and then did the same with always picking the cheapest territories. Such that the amount of defeated armies varies significantly while keeping the number of territories the same.
Because lets say that there are 1B armies to defeat and that there are 100 territories and there is a function that assigns armies to each territory similar to
https://en.wikipedia.org/wiki/Zipf%27s_law . Then if you take the lowest x% of territories you end up with some y% of all armies where x and y have some correlation that should be similar to the territory-AP-ratio we observe here by means of individual data-points.
And to quote Master Jz: "gives approximately". "gives approximately" is weak evidence. If it was that easy, why do we get "approximately" these amounts of APs? Shouldn't it be pretty much trivial for the game to reward "exactly" this amount? I just check with my current level and this formula almost fits but it is off by more than just rounding errors. xx.29 instead of xx.275 as the positions after the decimal point. For whatever reason I would get MORE APs than the formula predicts. (Side note: apparently the starting territory isn't counted either way, neither as "claimed" nor as "part of the level", so always subtract 1 in this formula, but even leaving the 1 in doesn't improve the closeness to the approximated amount)
But hey, as krinid said, aborting at that stage isn't logical IF YOU INTEND TO FINISH THE LEVEL within this ascension. If you only collect as much AP as possible to ascend and to get all the levels reset, that's definitely fine, too.