Somewhere I have a very old copy of Railway Modeller (1956) which gives all minimum dimensions - the problem is, I can't lay my hands on it at the moment as all my magazines have been moved during a recent domestic upheaval!
However, something to bear in mind is that if the tracks are on an embankment, the fencing is usually at the bottom; if in a cutting, then the fencing is usually at the top. Note I say usually as there are exceptions to every rule!
For track on the level, I usually lay fencing splines so that the spinning circles just touch the outside of the nearest rail. It may not be correct, but it looks OK to me!
I'll keep looking for that magazine as I refer to it frequently!
Ray