300 wm head space gauge

[pappy83]

I got my 300wm head space gauge and it is quite different then my other gauges 7mm saum, and 308. I understand because it is belted but do I use it the same way i do the other gauges it is really short. Also my fired cases from my factory savage rifle are 15 thousands longer measured at the datum line then my new brass I only bump them back a couple thousands when i resized them after firing so they are not stretching so much like when they are new. Should I use a fired case from my current rifle which shoots great to get headspace for the new rifle and then just bump my shoulders back when i reload a couple thousands when I reload them.