I'd go up to 50%. I'm not a "black out all your lights" kind of guy. I live in Canada, I'll take all the sunlight I can get. I just want some sunglasses for my car, nothing crazy.
There are plenty of days I go to work in the dark and come home in the dark in winter. I don't need to block out 95% of the visible light. But damn does the black interior get hot in summer.
Besides, heavy tint is a cop magnet and I heard talk about tint fails safety now. Might want to fact check that bit.
I'm planning on using that heat blocking stuff on my house's windows upstairs, too. Gets pretty toasty when the sun shines in, so why not?