Technically by the book you should stop 6 foot from any other train. The same is normally applied to buffer stops. But local instructions can alter this.
At kings cross, east coast stop at markers purely to ensure the tanking pipes line up. At FCC we sometimes use the markers but also go to 6' from the stops, driver preference really. On platforms 9-11 we stop right up against the 'stop' boards. They were put in for 365s with the aim that the train would touch them as it's a very tight fit for an 8 car. When you go in ontop of another train in 9-11 we stop 2' from the train, not 6. But then a single 313 stops about 10meters from the stops at kings x 9-11 on a yellow sleeper to ensure you arnt over the tpws loops when you change ends.