You didn't mention the caliber or pellet weight you are using so there is a lot of speculation coming. Gamo claims 1300 fps with a 4.7 grain PBA pellet. If that was true, that would put it at about a 18 fpe air rifle. If you were using a more 'normal' 8 or 10 grain pellet in that rifle, the pellet would reach it's in flight peak between 25 and 30 yards out then start dropping. At 18 fpe (if that is accurate) a 8 grain pellet would be going around 1000fps (which is pretty fast for long distance stability/accuracy for most rifles). A 10 grain pellet would be moving around 900 fps which would be better. If that 10 grain pellet is sighted in at 10 yards, it is going to also be above your cross hairs till about 45 yards (according to chairgun) where it will be back in the center again.
I'd suggest downloading chair gun and plugging in your actual numbers then click graph to see what the flight curve looks like. If you want to shoot accurately at 30 yards with that rifle, I think you'll find sighting in at 25 or 30 yards is going to be a better fit than 10. With all the above speculation, sighted in at 30 will have the pellet basically on target from 15-30 yards and about a half inch low at 10.
