I'm half decent at FS rocks, I got em pretty consistent, but hesitant to put em on any ramp bigger than 6-7'.
Once you get the hang of forming the trick and rolling away, I find I can curb the fear factor by making em quick. Stalling out is where I get sketched.
Keep the back leg cocked as you hit the lip, rock over keeping pressure on the coping with the back truck, smack the deck fast and loose, whip the hips around using the leverage of the coping and your back wheels, dive in