From what I remember back in the day the ball park number discussed for expenses was around 40-45% leaving a profit margin of approx 60-55% on self serves BUT that was back in the day say late 90's early 00's. A lot has changed since then obviously and the only thing we know is constant is everyone's rising cost i.e. utilities, soaps, parts, etc.
I would agree if your profit margin is 30-50% before taxes and deb service as Soapy mentioned you are doing good, anything over that and I'm jealous