It isn't hard but the intermdiate shaft is hard to position by the mark and tends to slip the belt, using the painless rotor-to-mark (very precise) check takes the error out of it. Guaranteed. Your geometric construction may work too. But don't make the beginner's mistake and expect to turn the engine over once or twice and have BELT marks come back around in synchrony.
And you can tell instantly after a bad belt change if you slip a tooth by the characteristic lag in acceleration. I learned the hard way too.