I've had a couple of folders that needed focus adjustment.
I remove the focus ring and focus the front element on infinity on a groundglass at the film plane. Then I tape a strip of paper with marked lines 0,5 - 1mm apart, with one of the middle lines exactly in front of/under the focus mark on the shutter, and take a series of test shots on film. The first with the focus ring at the first line, the second at nr. 2, then nr. 3 and so on. Develop the film and evaluate the negatives. If negative nr. 7 is sharpest, then I turn the front element to the seventh line, remove the paper, put the focus ring back on and fasten the screws.
I enclose a picture of a front element with the strip of paper taped on it. The mark on the lens mount is exactly where the groundglass says infinity is. It's corresponding to the line nr. 4, but the perspective is fooling us on the picture. The camera here is a 6x6 Welta Weltax, so I would have used 16 lines instead of 8, but I just took the paper I had for illustration.
The reason why I'm not satisfied with just adjusting the focus on the groundglass is that the film bulges somewhat, so taking a test roll will make the adjustment more exact. I'm very happy with my Zeiss Ercona after adjusting the focus. It's an excellent picture taker.