Strelok Pro use with air guns

I used 2.95" that I measured on my Impact. The (diameter of the tube + the width of the barrel)/2 + the distance between. I measured everything with calipers. Probably overkill. I'm pretty sure you can eyeball it with a tape measure and it won't make that much of a difference at a distance. 50 yards = 2.95" - 3.1 clicks, 3.0" - 3.0 clicks. It will make a more of a difference up close.
 
One time I used this tinfoil measurement with the left overs from making the hat..

I noticed that there is (still) quite a lot of parallax error or something else going on, since I made 3 shots which all landed separate. Or it is the fact that I am such a lousy shot that making a group at 50cm-1m is hard enough. (3-4 feet)

Maybe a longer distance would help?

As a side note, the scope height should be measured at the objective bell. This is where the light comes in = your eyeball "moves" here when looking through a scope.
 
Scope height should really be the distance between the optical center line and the tip of your barrel as that point is the minimum between the two and then from there the projectile is no longer moving perfectly straight but rather dropping.
That's the reason that a measurement at the objective is more accurate, especially with adjustable mounts. The trouble with direct measurements is they don't take the internal adjustment into account.

Yup, still the potential for parallax to get you with the tinfoil method I suppose.i haven't found it a problem but no method is perfect I guess 
 
Oh My God!!! I just discovered how to download and use Strelok on my phone! This is absolutely fantastic! It even has the exact reticle for my Sightron scope! Man I am loving this new toy... can't wait to use it with my range finder! WOOHOO! I have heard of this but thought it was only for powder burner rifles. :)

P.S. Thanks Macros for the tutorial on finding the true scope height.