Fine line vertical tattoo on a back

Reverend Father, the psalter you sent for copying has come back to me with its ruling spoiled. Brother Tobias pricked the margins with the wrong compass, so every column now wanders a finger's width to the left like a drunk man on the stairs. I have had to scrape the lines with a knife and begin again, and I am writing this with ink still under my nails and my patience thinner than the vellum.

I have set the boy Edric to the ruling board in my place. He is quick, but he does not yet understand that a single vertical rule must be drawn in one stroke, from the head of the page to the foot, without lifting the pen, or it will never sit true. I showed him on a scrap. He drew it three times, and the third was clean enough that I gave him the good ink for the afternoon. Brother Tobias calls this favouritism. I call it arithmetic, since the boy has now ruled eleven leaves without a blot.

Tonight I went to the cell where the old exemplars are kept and found Brother Tobias sitting among them with his pricking awl in his lap, weeping over the tiny holes he had made in his youth. He said every mark we set on a page is only a small promise that the line will hold. I told him the Lord had been patient with worse scribes than us. He laughed, the first time I had heard him do so since Lent.

Pardon the scrawl. The lamp is failing and the bell for compline will ring soon.