I would probably use your actual mileage for the trip you are making. So, if you leave work, then go to the medical appointment, then go home, count the mileage you accrue on your vehicle from the time you leave work until you arrive home.
The IRS is not going to come and check to see if you would have covered some of the miles anyway, such as the miles you would normally have from work to home.
This is just a guess on my part. If you keep these records this way, you can always reduce the miles at some point, if you feel that this approach is too aggressive.
Thanks, that is my impression as well.
I wouldn’t think that many people are audited on this (I haven’t known any) but would like some practical experience input on it.