Having trouble taking the club back on plane?
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Be careful that you don't start chasing your tail. How does your position at the top look? If your getting to a good position at A4 (or P4 if you prefer) than how you got there is largely irrelevant. Don't fix the esthetics of your swing, fix the functionality.
Here is a great thread for you to check out:
Steve Stricker appears way inside too, but this is largely due to the way he sets his wrists late, he hits the ball just fine.
Now, this is not to say that you don't need to fix your back swing, I'm not a pro so I can't make that determination but I highly doubt it's a priority piece for you right now. I would concentrate on the advice given to you here.
Take it from me, a guy with a hard head, you will improve MUCH faster by sticking to what the instructor instructs and not going off on a tangent of self-diagnosis. I struggle with taking my own advice, I have over-educated myself on swing theory and often get waylaid by my own theories and explorations instead of sticking to the program my instructor has laid out for me. It's fun and I like self-exploration but in the long run it is retarding my progress.
EDIT* Start setting (or cocking) your wrists a little earlier in the backswing and you'll appear "more on plane", but like I said, that may be a waste of time and effort that can be better directed towards your actual priority piece. Good luck.