understanding target_preserve_min

[DrTron]

All,
I'm trying to keep backups as defined in the retention policy. For that I have:
target_preserve 7d 4w 6m

What I don't understand is the target_preserve_min section in btrbk.conf. The manual says:

target_preserve_min all|latest|no|<number>{h,d,w,m,y}
Preserve all backups for a minimum amount of hours (h), days (d), weeks (w), months (m) or years (y), regardless of how many there are. 
If set to “all”, preserve all backups forever. 
If set to “latest”, always preserve the latest backup (useful in conjunction with "target_preserve no", if you want to keep the latest backup only). 
If set to “no”, only the backups following the target_preserve policy are created. Defaults to “all”.

So in my use case I would set it to "no", to only keep the backups as per the retention policy.
However, that results in no backups being run, only the snapshots on the source drive are.
If I set it to "all", the default, I think I would end up with no backups being purged ever. If set to "latest", it would purge all backups except for the latest one.

So what is the correct setting?

Thanks!