Always have a reference point for reverse parking, can't go wrong in most cases. For example, I pull up next to the car I'm gonna be parking next to on my left, and edge forward until the left edge of their car comes into "contact" with the back of my rear passenger window. Then I full lock it as soon as I start moving backwards, bang in the bay perfect every time and straighten up when I'm parallel to the bay lines. Since bays are pretty much parallel to each other on both sides of you, you can do this and then full lock to the right to park in a bay on your right as well. Obviously, this won't work for everyone, we're all different heights and our cars have differing levels on sharpness in their turning circles. Alternatively, like I used to have in my lessons, you can have a reference point in your front passenger window that you can line up with the line two lines away from the right edge of the bay you want to park in. Can be a Sharpie mark or something already there, as long as it's always there then you should be able to park perfect every time provided you don't make a silly error like steering too early/late.