I think the Ultradots have been produced in three tube diameters, DL; at least 2 diameters. The I" Ultradots are about the same size and weight as your Millett.
I agree with CTS that dot sights are not the right sight for long-range shooting, however it can be done... well! After sighting in my red-dot equipped .222/12 gauge Valmet combination gun at 50 yards with a .222 hand-load I had high hopes for, I decided to shoot a group at 100 yards to confirm the
actual trajectory agrees with the chart I'd printed from an online trajectory calculator. As stated in
Custom, Classic and Otherwise AWESOME AIR PISTOLS!, I chose a target with a large enough black circle to give a good sight-picture (center the red-dot quite nicely in the black circle). Thankfully lighting conditions worked for me this time; not always the case with dot sights.
After shooting the three-shot group, I pondered what result I'd be happy with as I walked to check the target. Decided a 2" or smaller group would be fine, and centered within 1-2" of my ideal point of impact (.70" high at 100 yards). To say I was pleasantly surprised at what I found would be a
SUPREME under-statement! Suffice to say it can be done. Also suffice to say I decided that an opportune time to end the accuracy testing!
BTW, when sighted in .70" high at 100 yards, the .222 hand-load shoots to within .70" of point-of-aim from 25 to 180 yards in the Valmet, virtually eliminating any need for hold-over/under with the dot sight. BTW, whether loaded with a Brenneke slug or 00 buck-shot, the 12 gauge barrel handles anything inside 25 yards...
quite handily! Here's the chapter alluded to above-
View attachment AP SIghts pdf.1624214703.pdf