Use 200 miles. And check every small town. A shop in a small town won't show up if you check 40 miles away, but shops within 200 miles of the small town WILL show up.
I often type in up to 30 small towns to find shops. If I type in within 200 miles of my hometown, results return zero shops. But if I go to each small town, I will find shops within 45, 60, and 90 miles. As Flash said, just keep messing around with it.
It's an improvement over when we had to enter by ZIP code.