Your cam timing mark is correct. I only use it as a guide as there are so many small deviations caused by wear and tear, belt tension, small differences in machining and aftermarket parts etc that at the end of the day you have no guarantee that it's 'perfect' - just in the ball park. Then when you do things that move away from factory original you've heading into uncharted waters (like the Weber - it runs differently and is sensitive to ignition tune, air flow) You have to experiment with it and each engine is slightly different which makes it that much more of a challenge.