Your Home Depot example is good. Generally, the IRS likes some kind of method for allocating the dual-use cost. In this case, I would suggest to keep records of your mileage and how much you spent at the destination, and then use that to calculate "equivalent mileage". For example, if your round-trip mileage to Home Depot is 20 miles, and if $150 of a total $200 purchase was a business expense, then the mileage to enter into TurboTax would be 20 x 150/200 = 15 miles.
