Forget the headspace guage, unless you just want it for "correctness!"
To headspace the 1919 correctly:
Make a reference mark on your barrel and barrel extension with a dot of white out at the point where you think headspace is correct.
1) pull the guts out of the gun
2) remove the firing pin - (very important!)
3) open the barrel 3 clicks from your witness marks
4) slide a live round down the t slot of your bolt and move the bolt forward in the barrel extension until the round is seated in the chamber
5) flip the assembly upside down with the bolt facing the floor and slide the breach lock down locking the bolt to the barrel extension
6) slowly screw the barrel in one click at a time until it comes to rest against the ammo, removing all slop - at this point the chamber is pressing against the ammo, the ammo is pressing against the bolt face, and the bolt face is touching the breach lock and there is no slop in the assembly. This is the point of perfect headspace for that brand/type ammo if all of the ammo in your lot is exactly the same
7) You probably want to open it up 1 click from that point to allow reliable feeding, but flip it over and compare it to the witness marks you made with the white out - if it's close you're ready?