Ditto! What kind of gun are you talking about, what kind of optic are you talking about?
If you are discussing co-witnessing of BUIS with a scope, there is no concern, since the focal distance of the scope will not allow the front sight to come into focus or obscure your sight picture while looking through the scope. If you are talking about a rail mounted Holosight or similar optical device mounted in line with BUIS, the optical axis or "centerline" of the lens must be higher than an imaginary line extending from the top of the front sight to the top of the rear sight. But without knowing what type of gun, sights, or optical sight you are referrencing, any advice is impossible.