<<the time from pushback from each stand to a taxiway entrance for each aircraft type will be a constant, as will the time spent on each taxiway.>>
Not a chance in hell!!!!!
Ancient Geek.. your profile reveals nothing but I suspect that you are not an aviation professional. Your suggestion of having timings to get every aircraft to the runway at a particular time is simply impossible. You surely understand that there is much more going on than just aircraft taxying? Vehicles moving around, work in progress possibly closing sections of taxiway, inbound aircraft waiting for a stand, sudden runway closure, weather factors, runway changes, etc, etc. I did GMC at Heathrow for 22 years and no two sessions were the same.