Don't bother to try. There are two reasons for this. First, the tape position at the start of the project will depend on where it was left by the previous project. It will not necessarily be at an end. Second, the autoallocation software uses the real length of the tape as measured on the first pass.
Tapes vary in length by several hundred feet depending on their history of abuse.
This will not happen when or where automatic allocation is not in use because the tape management could be messed up. This action is provided mainly to help the Mauna Kea site technicians who have a very long drive to change tapes and whose antenna is often scheduled in scans for which the source has not yet risen.