I would say ~50% in the front and ~30-35% in the rear.
I currently have ~35% in the front and ~5% in the rear.
Also, here are your local tint laws (California):
Windshield: Non-reflective tint is allowed on the top 4 inches of the windshield.
Front Side Windows: Must allow more than 70% of light in.
Back Side Windows: Any darkness can be used.
Rear Window: Any darkness can be used.
Your highway patrol police have different regulations as well:
Q: Can I put after-market tinting on the windows of my vehicle?
A: The main requirements for legal window tinting in California are:
- The windshield and front driver's side and passenger's side windows cannot receive any aftermarket tinting.
- If the rear window of a vehicle is tinted, the vehicle must have outside rearview mirrors on both sides.
If you don't care about legality, please disregard.
Factory is 22%...that's what they told me when I had mine installed. The front matches almost perfectly. I would recommend just letting a place do it...tinting is a huge PITA. Plus, most places have lifetime warranties...mine came with one for $68 for both windows. Can't beat that price for the time and frustration I saved myself.
You could very well be right. My grandparents live in Florida, and he purchases cars right off the lot, and his last two cars came with 20% in the rear and 35% in the front, but the same BMWs here (Virginia) have 50% in the front and 35% in the rear. I would need to get my brother-in-law to get his gauge, but you might be right on the 20%.