Can't preach about what should happen in deepest darkest, but in the Middle East we use the 15 degrees (established) 15 miles both ways to and from the VOR.
If my ever hazy memory is correct, that works out to be about 3.75nm between the two at 15 miles.
Back in Oz in the olden days, lateral separation was defined as 1nm between the possible positions of two or more aircraft.
So you build in all your tolerances and add 1nm to them...voila! safe!
Cheers
