I am pleased that
@Angela S found the answer she needed.
I like to trim off the excess after I have drilled the blank. I get square pieces with holes that I make into beads for my young nieces to play with. (Note: Choking hazard for infants!!)
I use an engineer's depth gauge to measure the tube glued inside the blank. Slide the ruler along the inside of the hole until it touches the tube. Pull it out, and the ruler shows you how deep the tube is. Place the depth gauge on the outside of the blank and make a mark at the end of the ruler. Sometimes I slide the pencil with the gauge to make a crosscut line, and sometimes I pick up a 4 inch square and draw the line using the mark. Use your bandsaw or small handsaw to cut slightly proud of the line ... duh!! (P.S. Save the ends to make bracelet beads for the young ones! If the trim is wide, then I may cut off several smaller pieces for beads.)
Mine is an old vintage Lufkin 510 depth gauge, but you can find new ones everywhere. Pay $90 for a Starrett if you wish, or $10 for a no-name model. It doesn't matter how accurate it is, because you use it only to transfer the depth of the tube from the inside to the outside.
Here is a basic one that I found:
https://www.amazon.com/Stainless-Steel-Engineers-Metric-Imperial/dp/B07B9VH493
Here is a set that might be useful to woodturners, and it includes a depth gauge:
https://www.harborfreight.com/6-piece-technical-measuring-set-94447.html
I bought one recently, so I just opened it.
These are the cheapest-made, stamped metal tools I have ever seen, but the depth gauge will work for pens as I described above. Frankly, the depth gauge looks identical to the one on Amazon above for less money, and you get calipers and other "tools" with it. I bought it for the three calipers, which are awful, but should (barely) do what I need for turning. Someday I will buy better quality calipers if these prove worthwhile.