We can get the tombstone period of our Active Directory by
$root = [ADSI]""
$ds = [ADSI]("LDAP://CN=Directory Service,CN=Windows NT,CN=Services,CN=Configuration," + $root.DistinguishedName)
But if the value isn't set it means we are using the default which changes depending on the version of Windows used to create the forest in the first place. If we have upgraded then the value may not be what we think it is.
So far I haven’t found a way to get the original version of Windows used to create the forest. It may not be possible in which case time for a rethink