LaserJet II printers are very slow at drawing lines and grids. If you simply want to rule across under a section, it is much faster to do this with line-drawing characters. All TrueType fonts have an ‘emdash’ which will make a continuous line when appropriately reshaped. APL normally maps the ‘upstile’ character to the emdash, but check the fonts in CharMap to be sure!

leaf.Flow txt
leaf.CharSep '—'    © Check the mapping!