Does anyone have an idea as to what the breaking points are? Cause i think this would help us all out rather a lot, as i have spent as much as 5 minutes trying to figure out what to run.
I don't know that there's not really a concrete differential, but there are indicators I look for. One thing that raises a flag for me is when my coastal cities (which have harbors and customHouse) have domestic trade routes generating almost as much

as intercontinental, foreign trade routes.
Another sign I look for (this requires enough

points on rival civs to investigate cities), is when rival civs' cities most profitable trade routes are with your cities. I consider the difference between routes with your cities and other rivals cities the strength of the indication. By this I mean, let's you share a continent with Cathy and Liz is another continent. The three of you share open borders and trade routes with one another. If you look at Cathy's coastal cities and see that routes to your cities generate more commerce than those to Liz's cities, you can conclude that Cathy profits more by trading with you than Liz, especially when she has a customs house in that city and your route still outperforms one from another continent.
In these instances, when you switch to merc, you and your rivals will setup new replacement routes. You'll probably loose some commerce, but the AI's with whom you were trading will loose more. The size of the gap between you the second best trader determines how much your former trade partners will loose after the switch. The trade is important. If your routes are the best, but there are 2 or 3 other civs with huge populations (that aren't running mercantilism) closing your routes will allow these other civs to scoop of the trade and replace you, without setting the global economy back very much.
Finally, I believe the demographics screen provides relevant information. If I have highly productive domestic trade routes, but am loosing out in the import/export category I start thinking about Mercantilism.
Note,

missions are cheaper when targeting cities in which you have a trade route. This cuts both ways. I'm not sure whether or not this affects automatic

spread.