vapourer is absolutely correct.
Golden Rule =
never never use anything other than a local QNH when operating below any controlled airspace (CAS) the base of which is defined as an altitude. In all cases the altitude concerned will give you a clue as to the local Transition Altitude which could well be above 3000 feet.
Irrespective of whatever pressure setting you are given by any ATC unit, if you're operating below CAS where the base is an altitude, set a local QNH to avoid infringing it because the RPS will always be lower than the actual QNH and so if you use it, you'll actually be higher than your altimeter indicates.
