In reverse, the torque resulting from the propellor rotation will always "throw" the stern of your C30 to port. It is a matter of physics and you can do nothing to prevent it.
The way I learned to use this reality to my advantage was this. On a calm day I motored close to a vacant mooring. Centering the tiller, shifting into reverse, and gunning the engine, I saw the stern swing to port as the boat began moving backwards. Repeating this a few times gave me an understanding of exactly what was happening.
I have a single lever shift/throttle control. I found that gunning in reverse then immediately shifting into neutral would minimize the swing to port and start the boat "coasting" backward. While in neutral, without the effect of the propellor torque, the boat could be easily steered to port or starboard.
As the speed falls off to the point where there is no longer any steering way, a quick burst of throttle in reverse gets the boat moving again, restoring maneuvering ability. An hour or so spent in this exercise will go a long way toward improving one''s boat handling knowledge and skill.
If I must reverse to get into a slip, I maneuver the boat so that the thrust to port, along with a little rudder action, will pull me in. For me, trying to do it any other way is an exercise in futility.
(When in reverse, the water pressure against the rudder acts to force the rudder hard over. Hang on to that tiller or wheel and control it to prevent the possibility of damage as a result of jamming the rudder against the stop.)