You can adjust the "Per SQ ft" price up or down depending on difficulty and adjust the "per stop" price up or down depending on trimming and cleanup involved.
I agree, you need to able to cover your cost per hour. The problem is with new properties it is often hard to know how long it will take. The SQftage is a measurable thing but mowing time isn't until you've done it. I have been trying to estimate times by comparing to similar properties, but that isn't exact either. If a formula can be put together with several of the measurable variables, an estimated mowing time/price can be calculated. I know a lot of people like to go with there gut on estimation mowing time or can guess mowing times just by looking at it. But others like to calculate things more exact, or need to break things down into measurements.