Thread: bullet and case comparators

Hi everyone,
heading down the rabbit hole of reloading and I've got the impression knowing your CBTO and headspace/caselength is important. I've been looking about and it seems there are a few of these tools about but they are quite expensive for a block of metal that rests against a bullet or cartridge shoulder. Also starting to learn how to use a metal lathe...just wondering if there is any reason why I shouldn't make my own?
As far as I understand it its the relative numbers that are important, not some absolute universal "truth".
As i understand it I need to know X, the length of my datum on the shoulder to the base of a fired case and then I need to set my die to resize my case to X - 0.002. The actual value of X isn't important. Likewise I need to know Y the CBTO that touches the lands and I need to set my seating die for Y-jump, but again I don't really care what Y is.

Just seems like a relatively simple lathe project which would be good practice and I dont want to drop $200 on things I could make and learn from. Or is it actually critical to measure to a specific (rather than constant) point on the shoulder, likewise on the bullet?