A co-witnessed dot will appear at the tip of the front sight when looking thru the iron sights. It's how these sights should be setup. Take your zeroed rifle and just position the dot at the tip of the sight and *** Poof *** the dot is zeroed too.
After that point look over the iron sights and just put the Red Dot of Death(tm) on target.
Takes a riser or a high mount to get co-witnessed.
Inexpensive Tasco red dot on riser:
Inexpensive aimpoint clone on high mount:
Expensive Aimpoint CompM (M68 COO) on mount and forward rail (original M4 Carbine)
And, no, you do not have to remove the front sight to use properly mounted optics. C79 (ELCAN) on standard CF-issue base.
-- Chuck