I would not charge per square foot but by the job. You need to guesstimate how long the job will take and charge enough to make your usual hourly rate or charge a minimum. If you don't have a minimum charge you should. We used to charge $100 minimum and that was years ago now.
I would second the motion to adopt a minimum price for any type of job that doesn't work at sq. ft. pricing. I have a $125 minimum in my own city; $150 for the next towns out; and it can go up from there depending upon how much travel distance and time are involved.