
There is a lot of info on this forum. Here is what you might be looking for in the FAQ sticky in this General PS90 Discusion section.
Check out the wikapedia link. There is a great ballistic graph there.
Have fun learning. There is A LOT to know.
Q: Is there an issue with having an aftermarket optic sit so high up over the bore of the rifle?
A: This can be a hotbed issue - especially among people who do not even own a PS90 themselves. Logic would dictate that the higher off the bore your optic is, the more variation you will have at distances outside of your zero. But, when one accounts for the fact that the PS90 is made to be a personal defense weapon, NOT a long range sniper rifle, this bore height issue really is a non-starter. You probably will not be shooting this gun out at super long ranges. And, within the stated range of the PS90 (200 meters - which is really stretching it anyway without a magnified optic), the optic height above the bore doesn't really matter much.
In fact, when the PS90 first came out, many people (including members here) even went to modifying and cutting up their receiver, so they could make their aiming optic sit as low as possible.
The stock optic on the PS90 is sighted in at 50 yards. At 25 yards, it will be approximately 2 inches low (you can see a ballstic chart of the 5.7 round, and more info on the PS90 here:
http://en.wikipedia.org/wiki/PS90 ). Truthfully after replacing my white ringed sight with an Eotech (which does sit a bit higher), I get the same variation. And, ANY rifle has variation when you shoot at different ranges closer and further than your zeroed distance.