You need to run two stored procedures to provide the db_size_in_bytes parameter needed by sp_iqestdbspaces.
Results of sp_iqestdbspaces are only estimates, based on the average size of an index. The actual size depends on the data stored in the tables, particularly on how much variation there is in the data.
Sybase strongly recommends that you create the spare dbspace segments, because you can delete them later if they are unused.