Seems to me that you are fairly paid. Don't get greedy. Square footage gives you an idea of how long the job might take but, if you are paid hourly, more or less square footage is not going to change that. I have had cleaning people for years. You don't charge extra for a service only performed once or twice a year. You are still being paid for it. If the customer only wants you for, for example, 6 hours, and an extra one hour job is added, then an hour of work elsewhere will have to be eliminated. If they want you for 7 hours a couple off times a year, then you work those hours and you are still being paid for the extra hour - for the extra work you did. You don't get more money for doing it, just the regular pay. Again, DON'T get greedy. You might find yourself being told to not come back.