If shots are low with the turret at maximum (up) elevation then you need to lift the back.
If shots are high with the turret at minimum (down) elevation then you need to lift the front.
However it's unusual for an air rifle to run out at 25m, it's normally the much closer ranges like 10m which give it an issue, especially when combined with a high scope rail. So it might be worth looking at why that is, because even with a high or low rail you should be within the scope's range of adjustment off the bat.
A rail or adjustable mounts is the best solution. Shimming has been found to put stress on the scope's internals as it bends the tube.