Distance from Central isn't a suitable proxy at all if the journey doesn't involve a transfer at Central. Try say, Penrith to Kingswood + 1 bus - you'll clearly need a MyMulti 3 for that rather than the MyMulti 1 that Opal or Not says.
The handling of MyMulti tickets should actually be pretty simple:
Distance to Central can be computed even if the journey itself does not involve Central. And again, I can only use zone numbers if I have a mapping of stations to zones. Volunteers? ;) It's actually not that big a job: if I have zones 1 & 2, the rest can default to 3.