Interesting. So it is low both when you're at too close range and as soon as you go past your zero distance?
Here is what I'd say, and I'm not sure how helpful it'll be but here goes anyway:
A ballistic calculator is just an educated mathematical guess, the outputs are only as good as the inputs. You probably have multiple incorrect inputs. Part of the process of setting up a rifle for real precision shooting is a whole lot of data gathering to ensure your calculator inputs are as absolutely accurate as possible. There are two ways to approach this. You can go from easiest to hardest, starting with things like scope height over bore which can be measured with calipers, lots of muzzle velocity testing to ensure your input (which is just an average) is as accurate as possible, etc before moving onto the mid-difficulty factors like accurate range, slope angle, temperature, barometric pressure, etc and then finally onto the most challenging ones like calculating your pellets BC and gauging wind.
The other strategy is to just go to a bunch of different ranges and shoot groups and record your offsets and that day's atmospherics very carefully. From there you can play around with strelok until it spits out numbers that match. It is sort of a backwards strategy, but there is some necessary give and take when tweaking your calculator anyway, so if you think the problem is really from one egregious error, this might be a way to highlight it. *shrug*
