On the Camshaft Alignment method verses the 2mm Method Brian Calrlton had a thread on that and I believe he said he had done both and that if you line up the markes correctly it is extremely close.
It is certainly good enough to give you an idea if you need to use the 2mm Method.
The main thing is that it is an easy check that you can do when you adjust your Valves of for some other reason have the Valve Cover off.

It also makes you wonder why the Alignment Marks are OK to use for assembly but not OK as a quick check.
I do not recall the FSM saying that after you did the inital alignment with the Marks that you were required to do the 2mm Metnod.
Perhaps they are assuming during a rebuild new parts would be used.

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84 300D, 82 Volvo 244Gl Diesel