A good industry standard for direct Mail is 1-2% response rate. But I'll tell you its been a long time since I've seen a 1% response from my mailings. So at 1% you should have gotten about 4.75 calls. That might seem a little low...
However... You are marketing a service that is VERY unique. I don't know how it is in Canada, but there just are not that many stumps that need to be ground down. For example if you did a flyer for "grass cutting" lets say, you know that just about everyone who sees your flyer, (or if you distribute them you can make sure) has a lawn. You don't know if everyone who saw your flyer has a stump that needs ground down. So you have shrunk your market. The same would be as if you marketed dog fence installation. If you mailed to every home in a neighborhood, you have no idea who has a dog or not. Even though your flyer has other services on it, the only thing that customer see is the "stump grinding."
Since customers only look at these thing for a split second, they are not seeing anything else but the main selling point. If they are interested in that, they might look at your other services, but not unless they want the main service your offering on the flyer. Make sense?
So I say if you got one from 475 that is a good response. I think many people(and I'm not pointing the finger at you) don't realize how many flyers, postcards, door hangers, or whatever... it really takes to really get a large number of sales.
Don't get frustrated, keep putting them out!!!