As chuck above said, even a mount using a lower 1/3 witness, if your irons are sighted in, adjust the dot to the irons and it will be on target.

Examples...

This is a sight picture using the Aimpoint T-1 as the primary sight. I left the front sight deployed to give a picture of what lower 1/3 looks like...


This is the same mount, with rear and front deployed, and looking through the iron sights. Note that the dot lines up with the irons still, as they are both sighted the same.



This is the Aimpoint T-1 in a ADM lower 1/3 mount..


and a Millet DMS-1 in a ADM recon, that is absolute. You can kind of get an idea on the height difference...



Both work, but with the Magpul ACS stock, for the pure dot sight, I REALLY like the higher mount. In the past, I preferred the absolute co-witness, but after using this, I'll be staying with it.