Focusing an Argus 21

Hey. I recently found at my local antique store, an Argus 21 Markfinder. This is my first, and probably my only, Markfinder I will get.
The problem with it is I'm not quite sure how to focus it! It focuses like a regular SLR, but I have no way of knowing if the focus is right or not. I have heard this model of camera be called the Argus "Guess-matic" because the photographer has no way of knowing the correct focus.

If there are any 21 users out there that want to give me a tip or two on focusing, that would be appreciated!

Thanks,
Jcorll

You measure or estimate the distance, then set the lens to that distance.

It helps to stop the lens down if you can to increase the depth of field so small guesstimates are minimized.

Easy to use if your good at estimating distances.

Around f8 the DOF will be similar to a Minox. There are a lot of tricks the Minox folks use, like you can calibrate your own interocular distance and use your eyes as a rangefinder.