I'm a tech dinosaur, so don't do spreadsheets. What I have is a contemporaneous record of mileage (written in when I get gas, and kept in the car), and the FIRST PAGE of CPI/report, whatever shows the address and POV of the shop.
However, I normally complete only 20-40 shops a month (4-9 shops might keep me on the road for 3 days on an 800 mile route); it would be a pretty clumsy method for those road warriors who think nothing of completing 40+ shops in a weekend!