Just got 20 all around and 30 on the rear (xpel prime XR plus). 30 looks nice on the rear. Had no problems with visibility at night out with the sides at 20, but the rear window (even at 30) seemed borderline dark. I personally would not go darker back there. Additionally with the 30, there's less of a contrast between the moonroof and rear window when looking upwards from inside the car. No regrets and recommend it 100%.
I had the unique opportunity recently to re-tint my rear window (the longest/largest piece of glass on our cars).
Originally my front side windows and top glass (above the driver) were tinted with 40%. Then all the rear glass, behind driver was tinted 20%.
Once my rear window was replaced I got a chance to see what it was like in original condition again - but the side glass still tinted.
I opted to go 30% for the rear and have preferred just that little bit more brightness in the cabin. I also tinted my front Windshield 70%.
If I could do it all over again this is the combo I would have chosen for decent privacy, heat rejection, and ability to still have a good view out of all glass panels.
All Ceramic Tint (which has worked nicely to keep cabin temps lower...arguably a negative effect in winter is I need to turn on the heat lol).
Windshield - 70%
Front side windows - 40%
Top glass - 70%
Rear glass (on piece of tint) - 30%
Rear side glass - 20%
I know it's kinda all over the place but in practice looking on the outside into the car, you really don't see much if any of a difference between these different shades. It's quite subtle on the inside too, I have to be looking for the differences.
Keep in mind I have a dark interior. I think with the white interior the tint differences are more noticeable.
I have 30% on the roof and I am very glad I did. Before tinting it, at certain times of the day when driving, the sun be hitting my eyes and I felt like I needed a hat to drive. With the tint, it is no longer a problem, even on the brightest day.