Best way: Find your cover and use it and a timing light. I don't know what your set up is, but I don't think going beyond 10 degs is a great idea. I've set mine between 0 - 15 and I found 10 to be the best all around setting.
Without the cover, I only have a "hack" solution. Rotate your dizzy until the engine is just about to die at idle. My guess is you are about -10 degs. Then rotate it the other way until you have a solid idle. You are probably about 0 deg now. Note how far you turned the dizzy (between almost stalling and a slow but stead idle) call this "x degrees" of rotation. Continue rotating it in the same direction and your idle speed will pick up and when you have rotated it another "x degrees" you are around 10 BTC. Then rotate it x/2 more degrees and your at 15 BTC. Total hack - but it should will get you close.
Right. It's the 3 prong metal bracket mentioned in the first post. A little hard to see, I cleaned up the timing mark on the crank pulley and put a dab of white paint on it to make it more visible. The longest prong is TDC and the smallest one is 10 degrees advanced.
Kirk