Originally Posted by Av8tr1

In "most" aircraft (not all) above the dash you may see three little balls in a triangle position. This is called a eye position indicator. They are a sight guide. You line up the back ball with the ball closest to you for proper seat alignment. Think of it (Liberal trigger warning, pardon the pun ) like sighting in a rifle. Front sight lines up with rear sight for proper shot placement.

Here is a video that shows you how the big guys do it. Been a while since I have flown something the size of a small cessna but as I recall nearly all had this.

https://www.youtube.com/watch?v=Rsn7j9ItoA4