That line runs under your bum and ahead, not towards the spinner. Imagine a line through your legs and straight ahead through some point on the cowling, which is probably right in front of you. It's not through the spinner as that creates an optical illusion of being straight. If you look far enough ahead as Ultralights said, you will find that it is keeping you centred. The easiest way to to line the plane up on a line, or if you are on grass to point it at something that is straight ahead, like the corner of a hangar, then get in and see where you should look.