Make things easy as possible. It would be lots easier to put a longer bolt in, than rigging up a pencil. Here's what I'd do -
Use the scale on the side of the pivotram to make some pencil marks on the armboard. Just get it close. It doesn't have to be perfect. You'll see the approximate arc with the pencil marks. Then figure out where you want to locate the arm. With this in mind, I'd then set the pivotram at exact distance with the bolt just scratching the armboard. Then scratch a small (1/4") arc where you want the arm. When you drill, the arc will disappear with the rest of the mounting hole.
You'll need a bolt long enough so that 1.5 to 2" can be adjusted below the pivotram. Sharpen 1 end with a bench grinder or electric drill attachment. If the bolt has a head, then sharpen off the head. Thread it back in from the other end. When you set the pivotram to scratch lay it flat on the platter. Adjust the bolt height so that it contacts the armboard. Lift off the Pivotram and turn the bolt clockwise (down) another 1/4 turn. Then tighten up the nuts at the exact distance.
Trying to rig up a pencil holder would be a PIA. You can't use the bolt because you need that to tighten the slider. Have fun, that's a nice tool. You'll feel great when you cut an armboard at perfect distance.
neo
