Well, I do know for myself, that every two hours either north or south is around or equal to, 2 weeks difference from where I am at. But that is with almost anything such as when mushrooms appear, blooming times for certain plants, when deer are in rut in the fall or when fish are spawning in the spring.
So no, does not sound extreme for me to hear that.

I've never heard that rigid formula, but I know that suburbs such as Olympia Fields & Chicago Hts are about a week ahead of the Northwest suburbs.

I've heard of that formula many years ago and agree with it. Remember though, that we are talking about simular elevation and no coastal or lake effects.

I live in 6b/7a. So we can check this out. Tell me some of the plants you have and I will match with the plants in my yard. We'll see how far away we are in miles and calculate!