is the Chairgun application, available for Android or Windows PC. It's by the Hawke company and they no longer support it, but it works fine on my Windows 11 laptop.

But first, here's a quick thing to try - bend your barrel up. At least, that's where I'd start. Take off the scope, lay a straight edge down the compression tube and along the barrel. I'm sure you'll find that it's a "drooper". You can take off the barrel assembly off and use something to hold the breech block while you apply a little bit of pressure to cause the barrel to lift up. A Brit I know has a "magic gate post" with a hole through it that he uses for these operations. Air rifle barrels aren't that hard to get to bend so a little bit at a time and use a straight edge along the barrel/breech block to check progress. Reassemble, install your scope and try again. I'd center the cross hairs mechanically first, before trying to sight it in. Count all the clicks from bottom to top, divide by two and count half the clicks down. Same from side to side. Your scope is now mechanically centered. Reinstall on the rifle, try a few shots, rinse, repeat as necessary until you get it close to where you want it. At around 15 feet I'd not want it hitting more than a inch or inch and a half low at mechanically centered on the scope.

As for trajectory, it will depend on your pellet and velocity. DO NOT accept the manufacturers claims for velocity, most of them are "wildly optimistic". Run a few pellets over the chronograph and you'll be able to plug the info into Chairgun and come up with a good guestimate for where it's going to be hitting at "x" range.

Below is a chart I saved from Chairgun with the info from my ProSport. It's set up with a "point blank range" of 13.5 to 37.4 yards, keeping an 8.44 grain pellet at 762 fps inside of a 15mm kill zone at that range. In theory anyway.

Here's a link to the Chairgun "End of Life" page. You can scroll down and find the link to the "legacy" downloads. X-Act is another handy tool, also at "end of life", but also still completely functional.
https://www.hawkeoptics.com/chairgun-and-x-act-end-of-life.html

you'll need to figure in the height of the center of the scope over the center of the bore. That works out to 2.15" on my AirArms ProSport and UTG 6-24X56 scope. Your mileage (er, measurements) will vary.

I used a 3/8" blow dart broad head to eliminate several of the varmints. This sticks them to the wood and is silent. We eventually had vinyl siding installed. I have upgraded to a cold steel .625 blow gun. Sometimes one will attack my rain gutters. That will wake you up early in the morning.