I added 35% on the back and 50% on the front which makes the back more or less the same as the OE glass which is 40%. The standard, non sun protection e90 windows are at the legal limit (for fronts) as is. Because of this some places will refuse to do them. Not doing the fronts will make it look more OE anyway. The first pic I posted is a more realistic idea of what mine looked like.
I used a local guy to me, in Glasgow so no use for you. Plenty of places doing tinting now and forums like e90post, piston heads and detailing world will all give first hand reviews of people's work. There's also the national companies like Pentagon which is franchised but all have high standards if slightly higher prices.