I got the rear (side and back) windows tinted in the wife's IX35.
It was £180 and we went for 35%
TBH, maybe a little too dark as you can't see the kids waving to you as they drive off!
Suppose each car different though.
The tint place will be able to put 'swatches' over the window when you visit, prior to choosing.
edit, the lower the number, the darker the tint
Counter-intuitive, I know, but they class them by how much light they let through.
So 35% is less light coming through than 70%