If you have a guide with a dog, I'd just take any advice they want to give you. Your 20 ga 870 is more than enough gun.
Other than that, maybe talk with some local Maine Ruffed Grouse Society members after June/July. Nobody and I mean nobody is going to give you specific spots where to hunt but they will let you know how the hatch went. Ruffed grouse numbers are heavily dependent on the hatching weather. The next 3-4 weeks are crucial. We need warm dry weather. Cold weather isn't a season killer but isn't optimal, same thing with wet and warm, but wet and cold will kill a grouse season.
Also, check out the drumming reports late summer.
I always prefer hunting just when leaves begin to fall, I don't know when that occurs in Maine, it's usually 3rd-4th week in Oct in Northern MN/WI. It's better weather/scenting conditions for the dogs, the foliage isn't as thick, food sources are easier to pinpoint, and the woodcock are coming down in good numbers.