The percentage numbers are based on pass through light... if you put a light on it on one side and measure the other the percentage that passed through is the percentage of the tint.
When you buy tint there is a variance in the tint.... So if you buy 20% it could be 15-25% or something like that...
I run 20% on the backs and 35% on the fronts... Its nice day and night but I am a light sensitive person. 15% may be a bit dark for side windows at night but it depends on your sensitivity to light.
The windows at 15% will be almost impossible to see into in the daytime (will require face to the window with hands cupped around your eyes to see in).
Depending on the state you are in you need to be careful with tint... In my younger days when I did window tinting on the side, I was pulled over more times than I could count for window tint violations... Probably is the easiest ticket you can get, especially when you push the envelope with it.
In addition to darkness levels there is another measurement called reflectivity... (this is for mirror tint)... Some states dont allow reflective tint beyond a certain amount. Most officers do not have the meter to measure this but some will ticket you by eye ball... Here in VA I was told only 1 or 2 state troopers have this meter.
The darkness meter that officers have is USUALLY a one piece meter. Therefore it requires you roll the window down and they clip it around the upper half of your window.... What this means is the back windows that dont roll down cannot be tested with this type of meter. Again, as another warning SOME officers have a 2 piece meter to measure any window... again, in VA its only 1-3 state trooper as I have been told.
Point being, if you want to push it... do it on windows that dont roll down... lol... otherwise dont push it much or at all!