"The total mileage for vehicles in the depreciation schedule allow this, but the total mileage on the Vehicle/Employee Business Expense (2106) does not allow it."

"It's not unusual for me to put notes into the Ctrl+E detail fields, some of which are subtractions. For instance, real estate taxes might have a line item for taxes paid through escrow, and an offsetting line item for the portion reimbursed at closing."

I read these as two different issues. The first seems to be, "I want to enter the formula and see the Net." That's what I meant by "doing the math" in the field or not. The ability to enter into Detail section, multiple lines, using negatives and positives, to see the net result, is different.

If it shows you should Enter the Beginning Reading, and Enter the Ending Reading, then it should do the math. If it states to Enter the Mileage, then you did the math. I would never attempt to enter a negative in Enter the Mileage, as itonewbie points out.