This is actually not possible yet (AFAIK). We barely understand this on the river with more than one action remaining. Position/Stacks/Ranges play a role in true hand equity. In order to do this properly you need to do a game tree search to find out how the hand will play out, and how much you will win on average.
The key thing that you pointed calling with marginal showdown hands when there are still multiple rounds of betting. Chances are you wont be able to realize this equity because you will have to eventually fold before showdown, thus surrendering your equity. In reality you don't have, say, 20% equity here, you have 0%.
Generally on the river, it is better to have a nutted range; the higher your nut:air ratio, the higher your overall pot equity. We could be in a hand and both have 50% equity against each others ranges, but I could have 90%+ 'true' equity, just because of how the hand will play out.
This implies that on the turn (and flop), we wish to play the range such that the our true river equity averaged over all possible turn/river cards is maximized.
ANYWAY, this means that hands that always stay marginal like AK on 832 have significantly less value than a hand like Ts9s on 8s3d2c, just because T9s can make then nuts, but AK cannot (mostly). Hands which can swing from bad to good hands easily (like flush/str draws) are valuable and their equity can generally be realized. But again, realizing equity is a function of how the hand will play out, and your entire range, and your opponent range, and so on.
Sorry for the really vague explanation, hope it helps!
