Don't worry about the departure. The bigger the the departure, the more we look like full scale. Also, dont worry about what the plane does during the snap,regarding displacement of the line. It can move 30 feet to the left or right with no penalty as long as it is vertical before and after the snap. Wind Corrected of course. I like to give a little down elevator throughout the snap in the vertical to accelerate the snap which translates in less heading change. It also gives you more energy in the proper direction after the snap. UP!!! this is a heavy energy despensing maneuver. If you have a heavy plane, it makes it that much harder.
On the negative snaps. Show the departure first. I almost give it full down elevator before I commence with autorotation. A lot of Aileron Rate also helps with this, but Rudder is the biggest factor.
Practice with a purpose. Tackle one thing at a time. Don't worry about things too much. I still get it wrong!
Mark