Aloha William,
The Moss kit does contain a jet centering tool (atleast mine does, but it was purchased years ago). It is used on reassembling the older H type SU carbs. To use it, the needle is removed from the carb piston and the tool is put in its place. The tool looks dowel with peg on it, probably about a 1/2 inch long. The chamber and piston are then replaced in the carb body. After putting the jet assembly together and with the jet sealing nut loose, lower the carb piston so that the centering tool slides into the jet bearing and the the piston is resting on the bottom of carb throat. Now thighten the jet sealing nut. To verify the jet is centered, raise the piston and it should drop down with a "clunk" and the tool in the jet bearing. If it does, you now replace the needle and redo the "clunk test" and it should work fine. It is more difficult to describe than do, I've used it often over the years with good results.
Safety Fast,
Dave