Thanks for all the comments -
Yes, Dan, excellent point - when mounting a valve nest onto a horn, I like to draw with a marker a vertical line down the center of the bell stack as a reference. Then, as I get things on, occasionally check things with a 2' level. Obsessive, but it's my cross to bear....(Wade, I feel your pain.....

)
OK, so I took a good long look at this thing last night, and I noticed that when I put the bugle together, the top bow was tilted
slightly out of alignment (to the rear) with the bottom bow. Even a small (1/8 to 1/4", aka nearly imperceptible) deviation at one end will be magnified greatly by the time you get to the other end of the branch. Changing that alignment moved the contact point between the top bow and bell a little farther apart, which also pushed the branch farther away, making it line up better with the center line of the bell.......
After I straightened that out, I removed and remounted the valveset
again, and although I still think the bottom bow is about 1/2" wide (these horns were all handmade, and therefore inconsistent), I struck a much better balance with making the valveset parallel to the bugle -
BTW, I'm
really happy with how this horn plays, too...very in tune with itself, and the smaller bell flare (17 1/2") gives it a nice punch when you open it up.
Getting there, little by little - next task, the 5th valve circuit.....off to bed now....
"Even a broken clock is right twice a day".