Well, just for reference, we had 30% installed on the fronts to match the OEM rear tint and it looks great. (my thread is in the X5 pics forum). If you want to go darker, go for it, but keep in mind, nighttime visibility gets harder.
I think the 30% is perfect, no darker than I would want to go... To each his own. The key is to get a quality material and a really good installer. Llumar FormulaOne Pinnacle carries a lifetime warranty. That's great for me.
__________________
2018 F15 X5d Carbon Black/Nutmeg Individual- Oil Burner Tow Rig
2013 E82 1M3 Carbon Black/Black- S65/DCT Track Toy
2013 F25 X3 35i Alpine White/Chestnut- German American Family Hauler
Past: '11 E91 335is, '11 E90 M3, '10 X5M, '14 328d, '03 330i Dinan3 ZHP, '04 330i ZHP, '12 X3 35i