Yessir, you are in the correct ballpark. When the throttle lever gets to the notch or is centered in the notch, the LOWER throttle shaft should just start to move. You may only be able to set it close, but not right on as there is usually wear in the linkage and holes the linkage fits in, thus resulting in the throttle shaft opening late. I have been having some trouble with that lately on my 1957 and 1960 3 hp’s. The lower shaft just starts to move about 1/4"- 1/3" past the notch. They seem to run just fine at this setting. 1/4"-1/3" is not critical in my opinion, but is acceptable, IMO.

An alligator clip with some of the teeth ground out and a thin rod attached to it makes a good amplifying tool to detect when the butterfly is just starting to open. To get an accurate reading, it gets clipped on the butterfly shaft at the opposite end of the linkage to allow for play in the linkage.

On the three horse motors, the bottom of the cam can be bent out a bit to help get the synchronisation spot on.