I personally wouldn't put any tints on the front.
But about 3 months ago i went into the local police station with my brother as i had a bet with him. He thought you was only allowed 70% on back 3 windows or something silly. I said no you dont you can have whatever on the back 3 because legally you dont even need to have them (vans lorries etc).
Went to local police station and they told me the back 3 can have anything, the front sides must let '70%' light through, and the front can have nothing in the area the wipers clear.
It was a woman though, and she didnt seem too clever about cars. So she could of got it wrong?
On the other hand, my brother has had illegal front tints (dark smoked) for over a year and been pulled silly amount of times. They've not checked them once. Depending where you live you can get away with it. But i wouldn't recommend it.
If anyone decides to put tints on front two, please dont come and make a thread moaning about it, like the guy in general chat the other day. Lol.