[Dovecot] resolve mail_home ?

I now have "mail_home = /srv/mailstore/%256RHu/%d/%n". Is there any way of asking dovecot where a user's home directory is?

It's not in "doveadm user":

$ doveadm user -f home janfrode@lyse.net
$ doveadm user janfrode@tanso.net
userdb: janfrode@tanso.net
  mail      : mdbox:~/mdbox
  quota_rule: *:storage=1048576

Alternatively, is there an easy way to calculate the %256RHu hash ?

-jf