There are a lot of ideas floating around about what’s a good swing plane. Most golfers today are coming in too steep, which results in an over-the-top move and a shaft plane that is vertical. Others come too far from the inside, shallowing the club and reach impact too flat.
An easy way to find the correct swing plane is to put a ball on either side of an alignment rod. If you tend to get too steep, work on hitting the ball on the inside of the alignment rod, and if you tend to get too flat, hit the ball on the outside of the alignment rod.