there is actually a number of free ruby scripts that allow you to eather mark camera location or even tell you the distance to a particular object, this takes out all guess work of setting the distance in vray
CameraDistance.rb - Returns the distance to an object in the direct line of sight.
cameraLines.rb - Connects all cameras of all pages with lines. (needs more then one page)
There is also old Film & Stage plugin that kind a works in sketchup 7, allows you to mark camera location, and do all the measurement.