Walter Horton wrote my dad a letter where he mentioned that Joachim had a computer punch card on his windscreen to show the proper lead on a crossing shot. Given an average speed and shooting distance some mathematician figured out the lead required and put holes in the punch card as a reference. Because of his desire to be the top fighter pilot he didn't share this trick with anybody.
It's notable that some AH pilots learned to do the same thing with cockpit frame references.
Several books about the RCAF/RAF ace George "Buzz" Beurling, make reference to Buzz using marks and lines he would make on his cockpit windows as well. He had created his own geometric aiming system. Beurling only spoke/taught this system to a couple other pilots, and his C.O. in Malta was one of them. Right after learning said system, the CO shot down 2 109s and an Italian aircraft on his next flight.
I'm not surprised that Marseille was using similar methods.