Ozexpat -
The arcs I used to fly had a specified VOR radial that one had to cross which signified when the turn on to Final Approach Track / Localiser could be initiated.
With this printed on the plate there was no (approved) opportunity to start the turn early or late at pilot's discretion, so one was at the mercy of whatever speed the arc and final approach turn was designed for (again not printed).
Does this differ from your experience?