So if I am correct, (and David G. will correct me if I am wrong), if you specify 30 days retention and 45 versions, all versions older than 30 days will be purged even if there are only 5 versions in storage.
If you had a 90 day retention and the same 45 versions setting, as soon as Version 46 is created, which could be on day 46, version #1 would get deleted, even though it is not yet 90 days old.
I don't trust/like the " #of versions" setting for this reason.
We sell 90 days of version recoverability to out clients for file/data backups. For Accounting and law firms that only touch customer files once per year we sell an extended retention period for a very small uplift that provides 15 months of versions. As you would expect, Image/VHDx backups are different.