I use a GLock OEM rear sight and have put an Ashley XS Big Dot night sight on the front only. Their literature shows holding the front sight atop the rear sight, but they show it using their rear sight which seems considerably lower and sloped. Since putting the XS on, my POI has gone a bit low, and rather than remembering to put the front dot way above the rear sight, I considered putting a lower rear in place. But the 6.1, 6.5 and 6.9 are a bit confusing to me. I got pictures of the different sights, but what do the measurements mean? Looks like the 6.5 is standard on the 9mm (I'm using a G26) so I figure a lower sight would be the 6.1, no? But looks like the 6.9 is supposed to be lower. Can anyone decipher this sight-height mumbo jumbo in case I decide to replace the rear sight? Thanks in advance for any help!