Not sure what the "top of the T" might be.
With a co-witnessed red dot sight the dot should be at the tip of the front sight looking thru the iron sights.
The use of a "high" mount is normally required to co-witness the dot and irons. Two (oft posted) photos show a higher mount. First on my parts box Tasco with a riser rail, and second a clone aimpoint with high cantilever mount.
Once mounted and zeroed you no longer look thru the irons if the dot is working, just acquire the dot and pull the trigger.
-- Chuck