Ketch,
You can improvise the seating depth gauge. Search the forum for numerous methods of using cases for the job. Probably the most common is to sacrifice a case by splitting the neck from mouth to shoulder with a hacksaw, a Dremel cutoff wheel, or a slitting saw. Clean up the burrs. Press the slit part together enough to hang onto a bullet with a bit of friction. Seat a bullet of the type you are loading in just past the mouth. Push this into your chamber by hand until the case shoulder meets the chamber shoulder. This causes the throat of the chamber to seat the bullet. Use a cleaning rod coming in from the muzzle to push this dummy out without disturbing the bullet position in the case. Measure the COL and call it maximum for singly-loaded rounds in your chamber. As long as you load that length or shorter, cartridges will fit into the chamber.
Work up a load with the bullet out far like that, using charges ten percent smaller than the normal load range. That will compensate for the pressure increase caused by making throat contact. Once you establish a best load there, try seating the bullets to 0.02" shorter COL and then 0.04" shorter, and so on until you find the right seating depth for the smallest groups. Once you have that, you can return to adjusting the powder charge to shrink them further. Sometimes it turns out throat contact was best for your gun. Sometimes the best spot can turn out to be much further back; even an eighth of an inch or more. This depends mainly on the gun and cartridge and bullet combination. Choice of primer and powder affect it, too. Many find the best COL in that first step back, but it isn't always so. Many guns turn out to have two seating depth sweet spots: one long one for single-loading, and a shorter one suitable for magazine feed. You just have to try them and see.