For codes designed for length 341 (the first length enough to support
512 bits of data):
* correct 1 error = 3 checksum characters
* correct 2 errors = 7 checksum characters
* correct 3 errors = 11 checksum characters
* correct 4 errors = 15 checksum characters
* correct 5 errors = 19 checksum characters
* ...
* correct 7 errors = 26 checksum characters (~ length * 1.25)
* correct 13 errors = 51 checksum characters (~ length * 1.5)
* correct 28 errors = 102 checksum characters (~ length * 2)

So it really boils down to a trade-off between length of the code, and
recovery properties.

At the risk of making the proposal more complex, I wonder if it might be better to support multiple checksum variants? The trade-off between code length and recovery seems to be largely determined by the user's medium of storage, which is likely to vary from person to person. I personally would probably be interested in the 51 or even 102 character checksums variants.