Hi Ferenc,
When you have same sets of members in WHAT, WHERE and USING, RUNALLOCATION treats it as XDIM_MEMBERSET. I.e., it allocates each member of that Dimension instead of total.
One of the options to avoid that is to add some DUMMY member that doesn't have any postings on it. Then use in WHAT all members while in WHERE remove DUMMY from that set.
Regards,
Gersh