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?

Look at the Hornady set. They list the datums. You can also gey the datum off the SAAMI drawings.
No trouble making them. Made one for an odd ball case myself. Measured the diameter at neck plus diameter at shoulder and divided by 2.
And they don't need to be thou accurate. As long as you are near the datum measurement and then you resize relative to your measurement. (I try for 1-1.5 thou)
Make them long enough to measure a loaded round.

Thanks mate. All sounds sensible, just wanted to check I wasn't doing something stupid/dangerous, unknown unknowns. Oh and super useful tip on allowing for a loaded round, thanks.

For base to ogive, measuring my Stoney Point inserts (now Hornady) -
30 cal = 0.298"
7mm = 0.274"
6.5mm = 0.254"
6mm = 0.233"

I also have a set of Sinclair inserts. They take the measurement slightly further back.
I also have some Redback Precision (Oz) inserts. They measure at a different point again. (They are cut using a chambering throating reamer to more closely match what is inside an actual barrel.)

There are more expensive option available if buying and people will tell you they are superior, but the Hornady or Sinclair inserts work just fine.

I've been using the Hornady ones, case headspace ones and cbto ones. Deem to work well and give that measurement so you know if you're seating/sizing consistently.