If it's wide open, the focus has to be perfect for it to work. You'll probably have a lot better luck with a lot more light and closing the aperture as much as you can get away with (say an f8.0 or f11.0), to minimize depth of field problems.
Without reflex viewing, your best bet is to go off focusing marks. That said, it's anybody's guess how well it'll work in a 60 year old lens. If the work is critical, you'll need to have the lens checked over and the back focus adjusted. If you have a digital camera that can be adapted to a d-mount lens (like a Pentax Q) you could test it with a tripod and a tape measure.
Off hand I don't know what the minimum focus is on that lens (My C8 has a SOM Berthoit zoom on it). It's probably printed on the barrel of the lens (it looks that way in the photos). Again, lenses never perform best at the extremes. You'll have better results focusing somewhere in between the minimum focusing distance and infinity.