Up the spring strength and adjust until you have the flattest trajectory possible.
Get a friend who has a spotting scope, or even just any form of magnified optics really, to sit with you at the range to spot you while you fire and adjust. He or she can tell you if it's staying flat, dropping short, has a hump in the flight path, etc better than you can tell without magnified optics.
If you have an 4x ACOG you should have no issues doing this yourself though.. that is unless you are one of those freaks with an ACOG red dot sight... O_o
Guardians of Asgaard
NO EGO, ONLY HARD WORK
Primary - Z-Custom VFC ZTW
Secondary - KWA ATP