The CTOT is based on your filed off-blocks time (EOBT), plus a nominal taxi time which is usually pre-set by the departure airport. For example, if you have filed EOBT at 0900 and the airport uses a 20-min taxi time, a zero-delay CTOT will be 0920. A 15-minute delay due, for example, to weather or capacity at destination [or any other reason, in fact] would result in a CTOT of 0935.
The taxi time might be varied by local ATC - for example, due to WIP, LVPs or congestion, it might take up to 30 minutes to get you to the runway. The taxi time might be increased in this case to avoid lots of missed slots.
Everything is based on you being ready to push back on your EOBT: that's all you have to do! Let local ATC sort out all the taxi timings etc.