I went with 35% all around as well. Illinois is a b***h when it comes to tint, especially outside of Chicago.For the majority of people, I would recommend 20% as the ideal tint darkness. It is dark enough to give you privacy and greatly cuts down on heat and brightness during the day but unless you drive in a poorly lit area is not so bad at night. In rural areas, even 35% can be hard to see through at night. In well lit cities, 20% is okay. Do keep in mind that if you have never really driven a tinted car before that even 35% will seem like a big change when driving at night at first. My first car was 35% and although the change was big at first, you acclimate to it and I later regretted not going darker. I'm in SoCal though so my winters are not nearly as dark as yours although your summer days are longer.
For someone who was very safety conscious or who drove in poorly lit areas, I would recommend 35%. 35% will still greatly cut down on heat but really does not add any privacy. 35% is also light enough that you may still wear sunglasses during the day.
Legal consideration - in SoCal such a large percentage of people have tint that it is unlikely you would get pulled over just for tint (but if you got pulled over for speeding or something else, they might cite your tint as well). The darker you go, the more you increase your chance of getting pulled over/cited for tint. I've been to Ohio before and much fewer people tint their cars there. So, you would stand out a lot more.
I currently have the equivalent of about 10% on the front and 5% on the back right now. I do not tint the front windshield at all. Excellent for privacy but I generally recommend that most people do not go that dark. There are two reasons. One is that at night or rainy weather, it becomes significantly harder to see. The other is that in the daytime, especially if you are driving towards the sun, there ends up being a massive difference in how bright the windshield is compared to all the other windows. It makes it harder for your eyes to adjust when looking in the rear view for example. That additional split-second in processing time makes driving incrementally more dangerous whether people acknowledge that or not. Of course, I could tint my windshield to mitigate the latter problem but I have mixed feelings about it and a tinted windshield in SoCal makes you a much bigger target to be ticketed by the cops.
There has always been cheap tint vs. good tint as well as good installers vs bad ones. I tinted my first car 20 years ago and it never peeled (if it peeled at the bottom corner of a window that rolls down, that is poor installation) or bubbled (that is adhesive failure) and still looked new (didn't turn purplish) when I sold it.