This is the way I do it. Ignore the points gap. Set the backplate so it’s in the middle of the slot.Turn the engine until the retarded marks align for the set of points that are on the master points plate. Attach a buzzer or light to the wire on these points. Adjust these points so they are just opening. You can check by rocking the crank back and forth, to get it as near as possible.Turn the engine 180deg onto the retarded marks for the other cylinder, set these points so they are just opening. If you try and do it with the gap set to the spec, you may not be able to get enough movement in the slot to bring the timing in.
This method will get you close enough until you can fine tune it with a strobe light.
Of course everything else has to be set, ie cam chain, and advance unit free to return to full retarded position.
This advice is freely give, and I will take no offence if you dismiss it as the inane ramblings of an idiot.