Above 35%, it really looks there is nothing on the windows.

To me 35% is too light. anyone still can see even at night while you are driving.
but it will give you enough shade against the sunlight.

I had 15% all around and took it off several month ago due to a ticket.
I was more than satisfied with the 15%. Noone can easily see inside while driving and when it's parked. Privacy FTW!! LOL