FYI tint numbers are backwards from what you'd expect. it's amount of light transmission that it allows. so 90% tint (which i've never heard of) would basically be just a dirty window. 50% is about the lightest you can buy anywhere. 20-35% is typical of the factory "tint" or "privacy glass" on the rear of many cars. 20% is what many shops install on the back window when there was no tint to begin with, and 5% is considered "limo" tint.
as for looking over your shoulder - you do the same thing as someone driving a cargo van or box truck or whatever - use your blind spot mirrors, and pay closer attention to what's going on around you to maintain a mental picture of where vehicles are immediately around you. no, it's not easy, but it's not too bad once you get used to it.