Have you done the normal stuff on your TF 89 like checking the stock screws, scope mounts, etc?
At the longer ranges on such small targets, I'd think you'd want a higher magnification scope with mil dots. I know on my TF 89 it starts to drop about a mil dot at 30 yards. Out at 70 yards I have to aim two dots high.
One of the other things mil dots allow is better compensation for the wind. In a "normal" side wind I aim about half a dot to the left or right.
I concur with practice. I'm headed out to Kansas in a few months to prairie dog hunt. A normal shot is 30 to 40 yards. However, I still sight my air rifles to 20 yards or so. I just want to be sure that when I get a good shot at the closer ones I hit them with ease. The longest one I've ever shot was at 70 yards on a calm day with my RWS 350. This time I'm taking my 350 again, my TF 89, and my TF 99. I'm actually going to take two rifles at the same time like I do for Coyote hunting. The 350 will probably be sighted in at long range, and the others in at the normal range (one of the TFs is going to be for family members to hunt with me).
Before I go out there though, I intend several hours of practice out at my friend Jim's farm. I can shoot as far as you could ever shoot with an air gun out there. It also allows me to practice in different positions, with shooting sticks, prone, etc. Prairie dogs are a LOT easier to hit than what you are trying to shoot.
Atchman