[flyingblind]

I haven't read all this post but I am with Lixma on this and he is right in saying you cannot replicate the way a 109 Revi sight works in real life on a flat computer screen and his solution of the sight ring floating outside the glass screen is probably the closest you can get.
To get an idea of the effect make a 3 inch square from card or paper. Draw a large gunsight ring and cross hair to fill the square. Sit in front of your monitor and holding the card in your right hand keep looking at the screen with both eyes open and move the card in from the right a few inches from your nose untile the centre of the cross is straight in front. You will see the cross and the computer screen in line and in front of you. Close you right eye and you will see the screen then close your left eye and your view will be obscured by the card. You can see that it would work much better if the card was clear glass or plastic, the sight was illuminated and it was given the correct focus for your eye.
Also, in real life it wasn't a case of peering through the gunsight or lining up as the optics of the sight meant the cross hairs or ring was floating in front of you eyes in the correct position regardless of head movement, all you needed to do was keep the glass screen between your eyes or eye and the target and manoever the plan to get in position. Those utube vids posted earlier in this thread show the way it worked. And sure, an experienced pilot could pull off all sorts of snap shots and the like but he was probably no more relying on tracers than taking careful aim.