Here's how to use the fish scaler on a Victorinox Swiss Army Knife as a rangefinder.
The Victorinox fish scaler has 8 serrations per 30mm, so each serration is about 3.75mm. If you hold it at arm's length then - give or take a bit of arm length variation - it will be about 2ft or 600mm from your eye.
With those two measurements you can work out the apparent angular size of each serration when held at arm's length: 3.75/600 ≈ 0.006 radians = 6 milliradians or mils.
A mil is a handy unit of angle measurement for range finding: it's the angular size of a 1 ft object at 1000ft (or a 1m object at 1000m etc).
So 6 mils is 6ft at 1000ft: therefore if a 6ft man matches the size of of 1 serration (6 mils), he is about 1000ft away. 2 serrations = 500ft, 4 serrations = 250ft, and so on.