I just eye ball it in the center. Perfect in the middle is not needed. I also use a piece of paper or a index card under the bubble so the white paper reflects the light. I cut the paper ends narrow on the end <=====> like that and squirt in the white Silicone in both ends of the tube, then use my thumb on the bubble to move it to and froe to make sure the vial is attached to the tube and vial.
PS: I scrape the plaster out with a drill bit that is a little under the side of the inside of the tube. Hold the drill by hand. I've also used a steak knife to scrape it out.
One it is dry and you put it back in the level castings, on a Starrett level one side is adjustable, So set the level on a level plane and flip it end for end and watch the bubble. and only adjust on one end of the flip. You adjust the bubble 1/2 the distance of the error. Then flip again, see where the bubble is, flip it back and adjust until your with 1 graduation or less. Be sure to wear cotton gloves when holding the level so the heat of your hand doesn't the bubble or casting.