The circular is not available on public domain.
Though they have a formula and as we can guess that it is time based no matter how much vacancy exists.
Clearly it helps them to recover train cost even when train runs under load, At any cost if train occupancy reaches 40% the train can be assumed as full according to current base fare. And on paper they have train too theoritically it can not be said that IR is not running Spl train during rush period.