Questor,
To answer your question: A LOT! :-D
The reticle on the scope is a standard duplex. What I did was simply as follows:
I first put the cross hairs on the bird. At the shot, I heard the pellet hit the pole. (The birds were always a little confused and looked down to see what the heck was going on.

)
From there, I simply raised the cross hairs a little more, and then a little more, until I actually hit one of them.
Now let me tell you, it is NOT easy hitting a small bird at 90 yards EVEN WITH YOUR MOST ACCURATE CENTERFIRE RIFLE. So you can imagine how hard it is with an air rifle.
The biggest problem with air rifles is that the slightest breeze can blow those little pellets right off target at those longer distances. As such, I had to wait on those super calm days.
And don't think that, once I got the hold-over right, that I was hitting them consistently. Remember, accurate shooting has to do with the shooter just as much as it has to do with the rifle. I would say that, on those 90+ yards shots, that I would hit about 1 out of 10.
Zachary