Trobin, measuring OAL, and COAL are different. Brian told you exactly how to determine you oal and with that you can get your COAL.
Here's what I do:
1. Make sure weapon is clear, and then remove bolt.
2. Screw the modified case down on the LNL tool, and insert bullet of preference into case mouth.
3. Carefully extend the LNL tool rod into chamber and slowly push the bullet in until it stops,( if this rifle was properly cleaned then this is the rifling stopping the bullet.) carefully screw set screw down on the LNL rod.
4. Carefully remove tool and bullet from rifle chamber.( sometimes necessary to push cleaning rod down muzzle to push bullet out of rifling)
5. This is your chambered round with bullet touching riflings, so measure with a set of calipers, without Comparator installed and write this measurement down.
6. Install comparator and proper caliber bushing to caliper, and then zeroe the caliper, with the comparator installed, and write this measurement down.
7. These two measurements will be recorded and you can easily compare the numbers with the book, ( as far as COAL) but for these purposes, use the zeroed comparator number.
8. Deduct, (just to start with, adjustments can be made during load developement) .010 from your comparator measurement, this number is where I start my data and build my loads. REMEMBER EVERY BULLET STYLE OR WEIGHT IS DIFFERENT AND STEPS 1 THROUGH 8 MUST BE USED.
9. Now with the bullet seated in modified case, remove Comparator and zero calipers, and measure COAL, (to make sure the bullet never got pushed back into case), and deduct .010 from this reading and see how it measures against book COAL's.
Hope this simpilfies the procedure you take to measure bulet seating measurements, adjust your seating dies accordingly. and relax and have fun.