I used the same marking method, but based it on Roman numerals. A "U" shape notch for one, a "V" for five and a square bottomed notch "l_l" for ten. "UV" would be number 4. Similarly, "U l_l" for 9.
If you do this, be careful to not go too deep with the notch; otherwise, the bottom will be beyond the film edge and you won't be able to tell a "U" from a "l_l". Also, remember that what you are numbering are the negatives, not the holder, so there are two different numbers coded on each holder.
This is the only way I have been able to take useful notes. I can positively identify each negative after it is developed. I can, also, positively tell which filmholder and what side of that holder is causing problems, if there are any.