Just get a holographic sight. Its the most common when it comes to a p90. As for how high up, it doesnt matter, just adjust your aim. When it comes to real steel, your bullet only impacts dead on, for so far. As for airsoft, you just want the bb going in the direction of your sights.
I can honestly say, most airsofters use their sights maybe 60% of the time, if not... less.
For the most part, its for looks. Unless your are using a dmr or sniper.
All in all, a simple holo or red dot is perfect for the p90