Airport CDM has muddied the waters somewhat.
Airports with CDM running will provide individual taxi times from individual stand to individual runway, varying by aircraft type, aircraft operator, season and other factors.
So a taxi time from LHR stand 511 to 27R in Summer for a BA 320 may be 14.79 minutes, but in winter might be 13.96 minutes, and for a BA 320 on stand 512 it might be 14.93 mins in Summer and 14.21 mins in Winter.
CDM also uses TOBT - target off blocks time. Airports differ in the tolerance on this time, some insist that you call ready at this time exactly to get push back approval, others have a 3 or 5 minute tolerance.
This should all be detailed in the airport's AIP entry.