Each airfield has an assumed taxi time from the gate to the runway. This can be increased in times of high congestion. Any delay calculation is based upon the difference between the EOBT and the Taxi time.
So at Heathrow, where assumed taxi time might be 20 mins, an EOBT of 0900 with a CTOT of 0920 will be attributed as no delay. EOBT 0900, CTOT 0930 is a 10 minute delay.
So in this case you should be ready to leave blocks no later than 20 mins before your CTOT.