For those of us who mainly use a straight but use a DE on a regular basis and have more than one, how do you keep track of how many shaves you got since you last changed the blade? Do you do it by how the shave progresses or do you put little tic marks in the blade or some other system? I just use the particular DE 3 times in a row daily and then trash the blade and so when I come back to it I know to put a new blade in.