The throttle setting should be irrelevant since the cam is keyed to the crank. All that matters is that the point gap us set when they are at the highest point on the cam. I usually set them at the idle stop or WOT stop just so the mag plate is least likely to move while setting the point gap. One or the other position usually makes access easier.

That said, I haven’t worked on any old Mercs yet so if it is somehow different I apologize in advance. By the parts diagram it looks pretty close to any other magneto so I’m assuming this is correct. That is how I set my OMC motors though. I know Carl liked things a little different so anything is possible.
-Ben

.Seems I’m really close the wAY it was describebed. Just watch the crank to see when the points open as far as opened, then set the next set of points 180 degrees out from the first. Very important to be 190 degrees out from the first set as the motor runs perfect 180 degrees out from the other. Next comes thw rest of the motor to see if it is correct too. Next comes the CC the chambers & ports, be sure to check reed valves for being straight. Bearings true & a zillion other items to be correct too, not counting ign & carb.