You will be fine with 20% (had this on my old ride for 5yrs and no issues, also have 20% on the Evo now with no issues). Just don't be dodgy on the roads and the cops will leave you be, well that's what I find anyhow
If you're worried just go for the 35% up front but I swear you'll want 20% after , I just didn't like the mismatch tint due to the privacy glass