So if you could just implement it as an option that would let game creators decide what the force surrender criteria are (in terms of % of armies controlled, income difference, % of income, etc.), that would solve the issue with different formulas for different situations.
However, I can see a ton of players exploiting this to farm games by setting really low criteria (say, "20% of armies") or something that they'll know they'll meet in the first few turns, so there needs to be a system better than Highlighted Settings for users to catch this. I like the new warnings in the game thumbnails; if we could implement something akin to that for settings like Forced Surrender (but even more noticeable, maybe a pop-up) that would help solve the problem.
