well, I was skeptical on mounting a dot....on a G19 I would get suppressor height sights, as far as co-witness? when I transitioned into red dot sights I read and heard so much about it.. 1/3 or absolute... to tell you the truth you put the dot on and once sighted in you focus either on the dot 100% and don't notice the irons, or you can focus on the irons. after just a few range sessions my eye was automatically drawn to the dot. I do occasionally now practice actually focusing on the sights... just in case. but the dot is so much quicker for my older eyes to grab and hold onto.
just my two cents and hope it helped a little.