It is -5 in each settlement for each settlement over the cap. So if you go 1 over the cap it is -5 in each settlement, if you go 2 over the cap, it is -10 in each settlement, if you go 3 over the cap, it is -15 in each settlement etc. So 7 over the cap would be -35 (7*5) in each city. But the game maxes out at -35. So 8 over the cap is still -35 in each settlement.
Yes, I checked the tooltip in the livestream and that is what it says. But the global happiness only goes down by 19 when Carl founds the 6th settlement, instead of 30. So maybe negative happiness in a settlement does not decrease global happiness (just does not increase it)?