It's all a crapshoot. You say 40-60 customers under contracts.....I am guessing for mowing? If that is the case, that might be high, but each case is different.
Few years ago I did 2500 postcards, twice. Got about 40 calls...not bad, but only picked up 4 mowing accounts. The majority of your calls will NOT be for mowing...just keep that in mind.
What kind of area are you targeting....income wise? I did some super nice flyers years ago and heard comments like "those are nice, I bet he's expensive"
Anymore, I prepare for the worst...lol. I know that sounds horrible, but that's the way I think now with advertising.