Bing carburetor throttle arm angle

I am by no means am expert but I have synced my carbs before and gave the process some though, so here's my opinion:

Mechanical sync process in MML tells you to check that vales open and close at the same time. This means the lineal run of the two carb arms needs to be the same. Some difference in angle is not a problem: the Bowden cable adjustment on the mechanical sync step is precisely to correct that.

By first fixing the idle screw relative to the totally closed valve position of each carb, and then fixing the bowden on the totally open position of both carbs, you sync the valve position to me mechanically equal on both carbs. This will hold as long as both have the same angular run from fully open to fully closed, which is always true (unless the valves themselves have been forced, and fixing that would require a carb repair).

Hope it helps!