Just use a sharpie and put a dot on the blade near the back for an SE, or near the center for a DE, for each shave. As others have stated you will know by the feel when to change the blade. But this will give you an idea of how many shaves you can get before tossing the blade. I can usually only get 3-4 shaves from a blade of any type.
Wouldn't you like to be a Gemocrat, too?
Sometimes I teach, but I am ever a student.
Life is not a journey to the grave, with the intention of arriving with a pretty and well preserved body, but sliding in broadside, a beer in each hand, totally worn out and exclaiming WOW what a ride!