Ok, but how do you get the stripes in the same place on all dozen arrows?
Good question.
I actually only measure where I place the masking tape on the arrow shaft where I want the cap to end; each length of cap is exactly the same on all arrows. Then, when I lay down that first band of paint, I have it end with just the smallest amount of overlap from the cap onto the arrow shaft; that covers up any "edge" or "line" that's there from spray painting the cap onto the shaft. (As I said earlier, if you lay down at least two or three thin coats as opposed to one heavy coat, it will minimize this greatly.)
As I just use the width of the brush as the exact width of my cresting bands, they all stay pretty much the same size/length -- as long as I make the same pattern on every shaft, of course.
For the little crest that I make down where my fletching is, I have sometimes layed a piece of paper on the surface of my cresting machine with "tick" marks with a pencil... That way, as I'm leaning over the top of the shaft and about to lay on a strip of paint, I can look down and line up where I'm about to place the brush with the pencil mark.