Y
The electrical boxes in the ceiling are 48mm high.

The plasterboard is 13mm thick.

48-13 = 35mm high furring strip.

But should I maybe add 1mm to avoid anything sticking out beyond the plasterboard?
That is, 36mm high furring strip.

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And, 1200mm wide plasterboards. Should I have 30cc or 60cc between each furring strip?
 
cc 30 between them and normally you take 22X70 or 22X95 approximately. You can let the boxes stick up a bit between the roof trusses usually.
 
Y
have set shortlings flat between the rafters... many boxes will also sit directly against the rafters... the question is what precision you dare to use, mathematically exact or count to 1-2mm and have all the boxes slightly recessed...
 
I would use a bit of countersinking myself
 
Y
I've had terrible anxiety after realizing that the old sloppily mounted noggins that the boxes were originally attached to were actually more correct than what I've done now.

They had mounted them about 1 cm above the rafter level. They were all a bit crooked, etc. I removed these and neatly mounted new ones flush with the rafters.

This means that I have essentially lowered my ceiling now by 1 cm.

From 240 to 239 cm...

Should I loosen my nice noggins and raise them... or have a lowered ceiling?

The catch is that no lighting will be centered with the rooms, everything will shift and look awkward...
 
239 is no problem in ceiling height.
However, if you go lower, it starts getting tricky with wardrobes for example. They are often around 2.36 high (if you want a ceiling trim, you have to run it around the wardrobes in such cases though).
 
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