A Civ Diplo Game has to balance two factors in war.
1. This is Civ - Warmongering, conquest and domination is a perfectly acceptable and honourable way to play. There is no intention that Diplo games need to be 'builder orientated' (though that is just as acceptable a strategy too).
2. This is Diplo - No civilisation should be crippled with the result that the game stops being fun for the player involved.
This means that there is a balance to be struck. War is costly for the aggressor, building up forces while other nations race ahead, so the victor should be able to make real gains from the war. But if they gain too much, the loser may be crippled.
Its impossible to make a hard and fast rule out of this, because that can just be exploited (e.g. a loser refusing to make a reasonable peace because 'the rule' will then kick in and save them). But as a guide, a 25% loss is an appropriate threshhold (loss being an overall measure, not just a city count - losing 3 tiny fringe cities may be much less of a loss than 1 major city). More than this may well be crippling the loser. But a loss/gain up to this probably is appropriate benefit for the victor. Again this can't be a rule - taking a small bite out of a neighbour every ten turns will cripple them, but having won a war against a neighour a millenium ago doesn't mean you can't fight with them again.
This concept of measured war is enforced in rule 7.
Minor change to rule 7 to make the link:
7. If 51% of the players believe that a war is no longer a 'measured war'and therefore should end, then it must end. This to avoid that civs will be cripled too much by a war. The terms of the end of war are in such a case: immediate peace for 50 turns. It is advised for both parties to agree on the best possible terms for peace before this happens.
Such an end of war must be organized in the org thread. (ie. players must announce their opinion regarding the end of a war in the org thread).
Such judgements over 'measured war' and an end-of-war vote are only OOC! Not IG (like: my ally is losing this important city).