Paul, That is what the hole in the tin foil is for. it changes the F ratio of the scope that is an old trick that they did with reflector scopes (telescopes). 

The biggest problem is the cross hairs and the design of the internals of your scope. There are glass optics that move up and down left to right that are inside the scope tube. but the cross hairs always look as if they are in the center even if you have the turret turned right to the top or to the bottom so you will not know for sure if you are truly looking at the center or not.  

Doesn't help you Kruz, but if you can somehow focus your scope on a point at point blank range, you will get your scope height.

Not sure how that method will give a more accurate measurement because you are limited to a number of factors. first off you have to make sure cross hairs in the scope are bang on center and that is floored because the cross hairs move up and down inside the scope which will give you a false height unless you are sure  that is also dead center. Then you have to work out the center of the pellet hole in the paper target you have just shot. Too many things can go wrong.

I have always done it the following way.

Outside diameter of the barrel % by 2 (if the scope is long enough) or measure the outside diameter of the breach and divide that by two.  and then the outside  diameter of the scope and % by 2. Add the gap between the scope and the barrel plus the two other numbers and you cant go wrong. i use my  Mitutoyo vernier calipers for this. To me this is a better method because the scope is fixed to the gun and that will not change