I have also wondered about whether or not to use the 'box speed' development time quoted in guides like Digital Truth for the EI I have calculated. I have normally done that and then adjusted the development time, if necessary, for future films. I think the quoted time for a particular film/dev combination at 'box speed' generally comes from the manufacturer of the film or developer.
If you got good results with the method you tried, I would stick with that speed and time.
Alex