Try these suggestions:
1. Using scrap materials (the same or similar to what you use for your pens) to do some self-training sessions. Thus avoid wasting the more valuable stuff.
2. Try turning training materials (scrap) to a size slightly over the desired finish size, using the bushings. You can do this by sight and by touch.
3. Get a set of digital calipers (DC). Use the calipers to measure as you turn your training blanks down very close to the desired finish size, knowing that you will sand as the next step. Learn how much material comes off with the sanding process.
4. Sand to desired finish size. You are learning to do each of the steps, including how to accurately use DC's.
5. Try this for training: take measurements using DC's. Do this using heavy and light thumb pressure. Notice the difference in you readings. Attempt to standardize your measuring techniques so you get the same result each time. There are probably Youtube vids for instruction.
I will stop here. The point is to devote some time and effort to learning each of the different steps in making your pens. The goal is quickly getting to the point of being able to achieve repeatable results...learning and practicing specific techniques. Before long, they will become second nature. Your pens will come out better, and everything will be more enjoyable.