If you're looking to add a checksum, why not use an existing format that does things like variable precision and proximity well like geohash (http://en.wikipedia.org/wiki/Geohash) and just tack on a checksum digit?

"ezs42cw4" becomes "ezs42cw4 f3" and my client can generate the checksum for me so I just have to verify, rather than enter the last two digits.