If possible, clamp the slide guide against the banjo and make sure the capo works well down the fretboard - then mark the holes. I found I had to angle mine a bit - more parallel to the string than the neck, because the strings get higher as they go down the banjo neck and you want to make sure the strings will not buzz against the capo arm when it is raised. (The “arm” has plenty of range to clamp down, but only raises up so far.)