I recently purchased a Norinco Dragunov chambered for 308.I sighted the rifle in with the included scope.PSO I believe they call it.Upon firing the weapon I notice that I am dead center a 100 yards,but two inches high.The meter adustment(elevation ) is set a 0 and I cannot turn it down anymore.The drum has two screws on top with a smaller black piece.How do I calibrate the scope to hit point of aim at 100 yards.Also,at 50 yards I am dead on.