It is okay to estimate your total miles, because you aren't deducting your total miles, just your business miles. Make sure that your estimate is reasonable.
For example if you usually drive about 25 miles per week for non-business purposes, you can figure it like this:
25 miles per week X 52 weeks = 1300 personal miles
In this example you would add your 1300 personal miles to your business miles to get your total miles.
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