To get a rough idea of the correct cutting angle, tilt the saw table, left side down, so that the tensioned blade meets the right line on the top edge of the wood, and the left line on the bottom of the wood. That angle, with the thickness of the wood in the photo and 1/4" wide ring, is about 20˚. If the wood were thicker or thinner, the angle would change.
If you go to scrollmania.com and look at the diagram for the Angle Calculator, you can get a clearer idea of what that diagram represents. Remember, the angle you are computing is actually the amount the saw table is lowered from its normal level position.
Hope this clears things up a little. And yes, there are people who just use the method in the photo to "compute" the cutting angle. If you're precise enough, it should work just fine, but I'd rather double check with a calculated angle. With a neat online tool like the Angle Calculator, there's no excuse for not being precise!