[Exim] Default quota value, when using lookups?

A quota can be obtained by using the Exim 4 "quota" value and doing an
ldap lookup:

quota = ${lookup
ldap{ldap:///uid=${local_part},ou=people,dc=test,dc=net?mailQuota?base?}
{$value} defaultuser}

Am I correct that this returns the "mailQuota" value for the selected
UID, and if "mailQuota" does not exist, it looks up "mailQuota" for the
user "defaultuser"?

We're concerned that this double-dipping just for a quota lookup might
be too expensive. For the sake of simplicity, only users who have an
abnormal quota are given a "mailQuota" LDAP field.

Is there a way to specify a default "quota" value to use, via a non-ldap
lookup, in case the ldap lookup fails?

Something like:?

quota = 10M if ${lookup
ldap{ldap:///uid=${local_part},ou=people,dc=test,dc=net?mailQuota?base?}
{$value} fail}

Thanks for any input!

Rich
richs@???