Place the two widest locomotives you have on adjacent tracks, then measure. This is your bare minimum. The NMRA creates a standard that is a compromise for many situations, and your situation may vary.
Generally, the NMRA standard for track spacing is wider than what you would measure on prototypes, even for straightaways. This is probably to accomidate the width of a human finger. If you placed yard tracks at prototype spacing, it may be difficult to re-rail cars.
Another consideration for track spacing may be turnout geometry. In several yards I built, the spacing came about naturally as I assembled turnouts into a ladder. Whatever the resulting spacing was between the diverding routes is what I used.
Kevin