For the RCBS Precision Mic, Hornady L-N-L, Sinclair Bump gauge or others, the calibration unit is a set of fired cases from your rifle. You measured the fired cases to get an average figure then set your resizing die to move the shoulder back the desired amount.
Two to three thousands for bolt rifles, three to five for semi-autos or something along those lines. See the instructions for what the tool's manufacturer recommends.
This process will minimize the working of the brass on resizing and should extend case life.
These cases may or may not fit in other rifles since the resizing is tailored to a specific chamber
The drop in gauges measure the headspace to the spec for that cartridge. Cases gauged in a drop in gauge should fit any rifle with a chamber cut to spec.
Note, bottle neck cartridge case gauges measure only shoulder position and over all case length. They are not a chamber gauge like the hand gun cartridge equivalent. So, a case could fit into a case gauge but not fit your chamber. Read the full description on the case gauges from the manufacturer's sites.
If you are loading for one rifle or segregate your brass, I would get the RCBS, Hornady, or Sinclair style gauges.