Here's a sight of a "lower 1/3" co-witnessed dot I found on the web. The bullet will strike within the dot no matter where it is in the window.
The easiest way to zero the M68 or other "dot" is to just look thru your zeroed iron sights and move the dot to the tip. Done. It's never necessary to remove the irons to use the dot and folding sights are expensive gimmicks for those of us without laser designators and night vision sights.
-- Chuck