If you incur an expense (mileage) in the process of meeting both personal and business needs, it is usually not deductible for business purposes. There is a presumption that the expense would have been incurred to meet your personal needs, whether or not there was an additional "business" benefit.
Individual facts and circumstances may, however, make a dual-purpose trip deducible.
In cases such as you described, I would ask "what was the primary reason for your trip to Home Depot - to buy business supplies or meet personal needs?" If I purchased $5000 of building supplies for my home repair business and $50 for personal home project supplies, then the entire trip would have been "business". Conversely, I could not justify a business purpose if I was at Home Depot for personal home project supplies and remembered that I needed a network cable for my home office.
However you decide to allocate these costs (or not), be consistent from trip to trip. For example - divide the mileage equally between business & personal.
