I have done something similar in the past, but used opaque Scotch tape. The opaque tape allows you to see through it, but the center hole is the only clear spot. The eye finds it naturally for shooting, but you don't lose as much general vision as you do with black or other dark tape.
Also, my shooting glasses were specially ground to have the optical center moved to the upper inside corner on my shooting side (left) lens. when my head is down on the stock, the best clarity is right where the sights are. Pretty neat. It was recommended by my optometrist, who is also a shooter.
ADK Bigfoot