You need to establish, for want of a better word, a datum line. I use a sinclair comparator tool. If you don't have one you stated that you have a lathe.
What you want is a bushing that goes over the neck and contacts the tapered part of the case halfway between the neck and the wall of the case. Using this bushing and a caliper you will be able to compare the datum line between a unfired case and a fired case. The difference in these 2 measurements will tell you if you have a head space problem occurring. they should be less than .005
If you measure the full length sized case and compare it to the fired case those dimensions will show you that you bumping the shoulder back to much. I would think if this is your problem you wouldn't have that ring at the bottom of the neck. Your not sizing your neck for its complete length. Look at your die and see if there is a ridge or shoulder in the top of the neck sizing area. if so you may have to trim the brass.
Regarding that situation I don't see anything wrong with that. As a matter of fact its a positive because when you chamber the round upthe unsized area will help center the bullet in the barrel. I do a lot of neck sizing and only resize my necks 3/4 of the way. I am using the unsized area to center the round in the barrel to increase the accuracy.
If you have time to do it twice, then you have time to do it once right and put your name on it