Special task for shutting down the OS, how to schedule it

Following proceeding is aimed

1. Chain of ATI tasks, executed sequentially
2. Last task executes user post command.
This commands schedules on-boot disks scan made by chkdsk
This command finally performs the OS reboot
3. PC reboots
4. on-boot disks scan by chkdsk executes
5. disks scan completes
6. OS starts up
7. special ATI task executes OS shut-down, this is its only one job
8. Job is done

Whole process has to be fully automated (batch files, ATI tasks user pre/post commands).

How to calculate the execution time of the special ATI task for the final
OS shut-down so it doesn't hit during some of chain tasks is executed?