Most dealers will do tints. You will need Ceramic tints(FormulaOne make a premium tint). The Metallic tints may intefere with the electronics, so BMW says. I would go all the way around. There is no legal limit in Ontario. It is all based on perception. If an officer pulls you over and thinks the tint is too dark, you'll get a ticket. There is no percentage or rating listed in the law.
Ontario - For the driver’s compartment, right and left, no film can be installed that would substantially obscure visibility from the exterior. Behind the driver’s compartment, any film can be installed. Someone correct me if I am wrong.
As a rule, 20% the darkest you can go on the front. Limo tints are 5%, but I think they are to dark and it will be hard to see out while backing up.