For comparison, I have the following settings:
* Daily backup (every day): Retain 2
* Weekly backup (every Sunday): Retain 2
* Monthly backup on the 1st and 15th: Retain 4
This way if a file/site gets deleted accidentally, I can roll back to the previous day (and go back a further day in case I've just missed the backup/rotation window). If there's a problem (such as a site exploit which was sneaky), I can go back to last week or a fortnight ago - and for safety, I then have the last 2 months available.
That's 8 backups being retained on the server offering a high level of coverage and flexibility for recovery - I'm not sure what you are aiming for for just one backup per week but keeping 2.5 months worth (and then an additional 4 months worth).
The "Retention setting" - as per the
backup configuration documentation - is the number of those backups actually kept on your server even after it has transported/copied them to a remote destination. If you are using a remote backup destination, then it's only worth keeping enough backups on the server for "rapid/urgent restores" - otherwise it's just wasted backup space (as they'll be stored remotely anyway). In your case, with the once weekly backups, it isn't worth keeping more than 4 of them as anything further than that is covered by the monthly backup.