I do not have the slightest clue about the formula for "unrest drop due to positive Dominion". I actually can guess what it is though: the exact opposite of the "unrest increase due to negative Dominion" formula.
A bit more seriously, the Dominion effect on unrest is not dramatic (between two turns): I would say it is around 0.5 point per point of Dominion, maybe 1 unrest point per point of Dominion, every turn. It also appears that Dominion-1 provinces drop two unrest points every two turns, instead of losing one point every turn, but I haven't done any extensive testing.