I have seen the marker idea before, but...
a) my lock gauge doesn't have the potential to run out of ink.
b) my lock gauge doesn't have the potential to bounce out and be lost.
c) my lock gauge doesn't have the potential to be rubbed or washed off accidentally, especially when caught in an accidental thunderstorm.
d) i don't want to wipe gasoline on my lens over and over and over.
e) JOG did this with a grease pencil, and, well, we know that anything JOG does we should all stay very far away from