I always use the distance of two switches (crossover) or a double crossover as my guide. At the club, we have the tracks spaced further in case one train derails, it won't affect the other train running on the parallel track.
If you go to the NMRA website they have measurements for this as well as many other other useful measurements.
http://www.nmra.org/standards/sandrp/s-8.html gives you the recommended track centers for both tangent (straight) tracks and curved tracks for the various scales and different curve radii.