Years ago, I put a piece of brl paper into my Perkins and brailled a full cell in the first cell of every line. Then I numbered them, in ink, from 1 to 25. I cut a strip about 2 inches wide. I keep it on my desk. In a case such as yours, where the computer layout doesn't give the exact measurements, you can just measure the distance on your strip between, say, lines 15 and 19.