Assessing rule for 16 m/m Camera Gun film

Gentlemen,
I do hope this is the right forum for this question and that you will be able to help me.
I have recently come across an undated Blueprint and a prototype unmarked aluminium slide rule for an “Assessing Rule for 16m/m Camera Gun Film”. I have no idea whether the slide rule was ever manufactured, or what it was for.
The calculation it does is:
The Percentage = (Total Marks x 100 ) / (Total Exposures x 3).
The scales are for “Total Exposures” from 1 to 2000, and “Total Marks” from 1 to 6000
The “Percentage” it gives is from 100 to 1
This begs a lot of questions:
– How was Camera Gun film normally assessed?
– Were such slide rules ever used?
– What does the “percentage” signify?
– What are the “Marks”?
– Why would the Total Exposures have to be multiplied by 3?
The Company from where the Blueprint and prototype came ceased trading around the beginning of World War 1. Whether this was from then or immediately after that War as an attempt at finding new products we do not know.
I can post pictures of both scale and rule if required
Cheers
peetop