As above, but, side windows are allowed slightly less light transmission (70%) compared to the front (75%) in U.K. Law
OEM tint from the factory is 70% all round (except the sunroof which is 20%)
So, as Paul says, from the B pillar forwards, your pretty much close to max tint level allowed straight from the factory.
I've got from the B pillar rear done at 20% on my Lexus and I would say that is ideal. I've got one of my other cars done with 5% tint from the B pillar back and find it a struggle to view out of the rear when reversing in the dark.