As I like to use science for good Lomographic reasons, let me present you the External Shot counter!
Sometimes, you cannot see the shot N° from the little window behind so I thought to make an external shot-counter. When you roll your film, you turn the wheel of a certain angle to fall on a correct shot position. This angle reduces as the diameter of the roll is increasing. Here, I used an empirical analysis (empirical means by experiment) to determine the good angles (more than 360° rotation before shot 12) needed to turn the wheel in order to make 12 separated shots on any film.
To make measurements of the wheel angles, I placed a white sticker on the wheel, used my Holga normally (by watching shot numbers in the little red magic window) and noted every position of every shot on the sticker. I did this for four films (Provia, Sensia, Velvia and a strange one I forgot the name). Then, I calculated a mean rotation angle over all films to obtain a mean measurement applicable to all films (I hope) by applying a third order polynomial regression. Finally, I produced an external shot counter with Photoshop, printed it, and stuck it on the Holga wheel. I tried it many times. IT WORKS.
As science is about sharing information, I put here a copy of the counter. Print it with the good size, stick it on your wheel and shoot. This is useful when you do 70 mm and you can’t open the red window (‘cause 35 mm are transparent, you know…)… And same thing for 220 film, note that after the twelfth shot, you just make a 360° rotation of the wheel…