Chaps,
I had devised a similar system on the Légère but, having set the ignition up to my satisfaction, I chose to mark the actual point of ignition.
It was fine at first but, over time, as the timing chain wore/stretched and wear in the Oldham joint increased (despite regular greasing), it became noticeably less accurate. I would suspect that to be less of a problem on an 11D engine with a splined pulley drive but the timing chain would still be a factor.
That said, I can envisage no other practical way of visually monitoring the timing except possibly by cutting a hole in the bell housing to see the flywheel – but I would certainly not endorse that.
B….