In Washington, I'm only allowed 24%. If the factory tint is as dark as stated here, perhaps I will just skip it.
Although with the sunroof shade open, it seems barely tinted at all, and I feel like I'm sitting in a fish bowl! I may end up changing my mind as I have always had the front match the back as close as possible.
https://www.garagechief.com/car-window-tinting-law-by-state/#ftoc-state-law-summary-chart