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I agree early on I think it is easier to figure out how much you want to make per hour and then estimate how long the job will take.
If you call around or look online on sites like craigslist, you can get an idea of what your local competitors are charging and then you can find where you want your price to be.
Then figure out how long it takes you to mow your property or a family member's property as a test. If you can then measure the size of the lawn with a surveyors wheel, you can then compare new properties to the old ones and know how long they take.
Or you can use the free lawn care estimate calculators
on this site here.
Let me know if this helps.