As long as your arm swing is free, any back swing height is fine. When you are using muscle to keep it low or using too much muscle to get the ball higher,that causes you to miss your target. If you just use gravity on the down swing you will have more speed + accuracy than if you just muscled the ball all the way through.
Some things to evaluate your game:
Look at how you finish at the line, are you in balance or do you fall off the shot? You want to be balance when throwing the ball. (look at my profile video, I'm falling off the shot. that is something that I do occasionally but thats what i'm working on to improve.)
Do you drift with your feet, drifting is OK, but only if it is consistent from shot to shot. If your drift is inconsistent, then you are out of balance somewhere in the swing.
Your set up before the shot, is that consistent? All great athletes no matter what they do always have a routine that they go through to get ready, you should be no different. For me, I pick up wipe the ball with a towel, dry my hand, and then set up for the shot, always trying to do the same exact thing. put my feet on the same spot, look at the same mark, set my hand position, and go. A good preshot routine helps take some of the thinking out of the game. It just lets you bowl.
It doesn't have to look pretty, just be repeatable.
Good luck!
