AS TIO says, we have very strict instructions of plus or minus 15 minutes.
Other airports due to taxi restrictions and length of time to the hold may have slightly different but our instructions are clear, we must only start up within the 15 minutes either side of the EOBT.
So far it has caused us little problem , I had expected a far greater difficulty but apart from the first few days with people getting used to the new procedure it now seems to work well.