A big part of success with rockets is consistency in your approach. That means taking pains to dive in at the same angle and speed every time. Speeding up, slowing down, or changing your dive angle will all effect where the rockets impact. Another factor is obviously going to be distance... Fire them from farther away, and they'll eventually slow down and dive in steeper.
As previously mentioned, rudder doesn't have much effect, because the rockets will self-correct and continue on your flight path, regardless of where your nose is pointing. That means, though, that you cannot use rudder to correct your aim, either... So, if you're giving any rudder input, and using the sight to aim, you'll miss... You need to learn to direct your approach without rudder, or at least fine-tune your aim without any rudder input, before you fire the rockets...
A final point to consider is how high your head is in the cockpit. Are you using the page up button, and then saving that view as default? If so, that changes things slightly. Especially if you use rockets in multiple planes, and don't have the up/down aspect of your head the same in each...
What I generally use as a reference for aiming is the space between the actual gun-sight and the top of my cowl. I fire my rockets when the target is about 1/2 way between my sight and the top edge of my cowl. For bombs, I drop them just as my target passes out of sight beneath the top edge of my cowl. That only works though, if my approach speed and angle are correct, and I'm not using rudder, and I'm the correct "same" distance from my target each time...