More than 45 grid areas are fully or partially included in Napa County. All of the designations begin with CM88, and end in two lower-case letters. At our latitude, the individual areas are approximately 4.5 miles wide and 2.9 miles high (and 5.4 miles along a diagonal).
You can get your latitude and longitude from Google Maps: just select your point on the map, right-click, and choose “What’s Here?”. Then, use this link to get your grid area: https://www.qsl.net/k7jar/Links_files/latlon2grid.html
(Map created using QRZ.com’s GridMapper)
Distances between grid references can be calculated from: http://edwilliams.org/gccalc.htm