There's nothing to get the painter as challenged as a self portrait. I usually tape some thin string to the mirror I'm looking at, to form a cursor. I align one eye with the centre of the cursor and the other along the horizontal axis. This makes it easy to adopt the same posture throughout. It's always worth looking at the portrait in a mirror to see how it looks. That's the simplest way to check how accurate it is. Another method is to take a photo from a similar angle, then superimpose one over the other in Photoshop, using a 50% mask. Obviously this only tells you how accurate the drawing is, not what a brilliant painting you have created! I checked this portrait using this method. The forehead is a touch too high, otherwise it's not far off. Unfortunately, like most self portraits, it has the artists almost insane stare. I do smile, a lot!