Hornady makes a head and shoulders gauge
MidwayUSA - Hornady Lock-N-Load Headspace Gage 5 Bushing Set with Comparator
It clamps onto your caliper and you take measurements close to the datum line on the case
Your case will most likely not expand enough with one firing to get a crush fit in the chamber. On a 300 win mag I own I get the following measurements during the firing cycle:
New case - 2.253" (subtracting the 2" for the gauge and the measurement itself is not important, only it's relationship to the other measurements)
Once fired - 2.270"
Twice fired - 2.272" (slight crush fit)
3 times fired - 2.2725" (crush fit and time to push the shoulder back)
So I neck size only with a Lee Collet for the first 3 firings and then push the shoulder back to 2.272" with a Redding Body Die for a slight crush fit. You will have to push the shoulder back each subsequent firing after.
Note that almost all the expanding happens on the first firing but the case has not been through it's full cycle of expansion. A caveat to this is that if you size the body and not just neck size then when the die squeezes the case body it will push the shoulder forward just a little. IME not enough to make it have a crush fit and it will not be consistant.
When sizing, set your rifle up in a vise and chamber the round as you go through the sizing process and it will tell you when you are developing a crush fit. Until you do, neck sizing will be sufficient.