Back when I started welding, I was having a hard time getting the hang of vertical up stick. I wasn't yet "reading" the puddle, and my weaved bead was undercut on the sides and lumpy in the middle. One of the instructors at the UAW/Chrysler tech center, Tom Soley, had this analogy: "Say you and 2 friends are in a pick up truck driving down a country road, and you're passing a bottle of whiskey back and forth. Who gets the drunkest?" Guy in the middle, he's getting two swigs for each pass! So the guys on the ends should take bigger chugs, and the dude in the middle needs to take a quick chug each time, to even it out. That is...hold the edges for a couple beats and pass the electrode through the middle real quick, and your vertical weave will even out nicely.