The thinnest line indicates the correct focus. It can be confirmed by looking left and right about 4 lines and checking those lines are equal sized.
I chose the one marked in green. And the yellow marked lined 4 to each side are the same size.
I almost chose the one immediately to the right of the one marked in green, but comparing the size of the lines 4 away from that there’s a big difference. In that case the line 4 to the right is noticeably wider than the one 4 to the left.
This focus test was done on paper, which is notoriously hard to engrave as it doesn’t brown well like wood and also isn’t perfectly flat, which is why some of the lines are not complete or are missing.