@PW: if your "BS" means what I suppose it to mean, well, err, I am really disappointed, I always respected you for a polite gentleman.
I gladly agree that Belgium, like most countries, knows the concept of a "Regional QNH" though,
AFAIK, the UK is the only country to base it upon forecasts rather than upon actually measured pressures.
But that was not my point. I do not want to argue the merits of the "regional" setting, it has its advantages and its disadvantages, I can see how it came to be though it might indeed be considered an outdated concept.
Neither was it the point of the topic starter. My point was and is that in BE (and, as far as I know, all over Europe except in the UK) the opening question has an easy and unambiguous answer: if flying at a place such and such, determine in what bit of airspace you are, and talk to the service (the one and only!) in charge of that bit of airspace. They will tell you what setting to apply; in fact, most of the time they will tell you the QNH on your initial call for entering their area. Basta. Everything clear. And you know you will have the same setting as any other aircraft nearby, if their pilots followed this easy procedure; as they must.