If the sign is behind a VRS, you need to determine the working width of the VRS, then put the sign just beyond that point, whilst hoping the sign isn’t so wide and on an embankment or cutting that you end up with some very long posts! If the working width seems excessive, there may be ways to improve it locally around the sign location, usually by installing extra fence post to toughen the barrier up a little – but check first of course.
If it’s on open verge then you’re now looking at the width of both the sign and the verge to see if the thing will actually fit. If it does, then personally I’d go with the old TSM1 distance(s) where possible. If it’s tight, do the best you can or possibly look to moving the sign, within the allowable tolerances, because the verge width only a handful of metres away might be wider.
All of the above bearing in mind the visibility of the sign to approaching traffic of course. Oh and a setback of zero or (god forbid) an overhang is never a good idea but I’d hazard a guess you know that already 🙂