How do you turn without bushings?
Turning between centers. There are a ton of posts on here about it. If you are experiencing concentricity (commonly but not correctly referred to as out of round) issues, this is one of the best things you can do to resolve them. Concentricity issues are when you have a perfect blank-kit interface on on side of the blank but are over or under on the other side (or you have a bad but not consistently bad fit all around) . There are many potential causes, but assuming your lathe is tuned and working as it should, nonconcentricity usually comes from the pen mandrel being bent or the bushings being unevenly worn. Turning between centers greatly reduces the chances of this happening.
If you are not experiencing issues with concentricity, and the bushings are working for you, I don't recommend changing things. If you are, get yourself a dead center for the headstock, a 60-degree live center for the tailstock, and a pair or two of decent calipers, then turn the barrel between the two centers, using the calipers to determine your final dimension. The disadvantage is that you can only turn one half of a two-barrel pen at a time, and you'll have to develop patience and restraint in your turning as it's easier to overshoot without bushings. The advantages are better fitting pens, fewer issues with pens getting stuck on bushings, lower costs because you no longer need to purchase (and re-purchase when they wear out) bushings, and better access to the ends of the blank if you want to do rounded ends as suggested by
@Mortalis.
To answer your original question, I don't turn to a specific amount below the desired diameter. I simply turn until I can feel the slightest bit of slop when I check with the calipers. The digital read-outs on both my sets of calipers have long since quit working, and chasing a specific measurement to that degree of accuracy is a fool's errand on a wood lathe anyway. Your CA coats are likely to be measured in thousandths of an inch, so you don't need to go much below your target diameter. As I said, I go just until the difference is perceptible with calipers. I imagine you could use the same strategy with bushings as well. Use your bushings only as a guide, and begin checking with calipers once you get close to your final diameter. Even without calipers, you could turn by feel just until the bushing is perceptibly larger than the blank.
