To be honest, I think there needs to be a major rethink when it comes to stewarding. I accept it has improved as they aren't looking into every slight tap or touch, but they need to apply the rules more consistently. I know it's a bit boring now, but look at the Rosberg/Hamilton clash in Spa. It was a small tap, it could of been avoided as Rosberg could of backed out it or cut across the grass but choose not to, as he stated. But most importantly, it effectively ended Hamilton's race, meaning a penalty should be applied. Anyway, I don't want to get too wrapped up in that, I'll get back to the issue of lines.
I used to race karts, and the rules were plain and simple, in black and white. If you placed all four wheels over the white lines, you were exceeding track limits and were breaking the rules. The white lines define the edge of the circuit and if all four wheels go over them, the driver should be punished - not too heavily, but enough to be a deterrent to stop other drivers from committing the same offence. The severity of punishment should then be increased depending on how much of an advantage is gained from crossing the white lines, effectively, a level system should be implemented. Level 1 simply crossing the white lines without gaining an advantage and Level 5 meaning crossing the white lines and gaining unfair positions.