It's really up to you and what set up allows you to see clearly. Personally though, I'd try and stay between 2 shades of tint - 1 for the sides and rear, another 1 for the front. Alternatively, you can have 1 shade for the sides and another for the front and rear.

What I've learned through the years is para di ka mahirapan, the front (or lighter shade) should be just a shade lighter than the sides/rear (or darker shade). This will allow your eyes to adjust a bit quicker than if the discrepancy is bigger.