The diagram was very helpful and the clarification of Chord being correct word for the use of a cord or string to measure it helps.
As an example, the way to find a degree of curve would be to use a 31-foot long chord and on a 1-degree curve the mid ordinate would be 1/4 inch.
In the above method of using a 31.25' chord for 25 mph on narrow gauge would mean the superelevation would be just a hair over 1/4 inch.
If the ng track curve was 4 degrees and you wanted to run 25 mph on it, the superelevation would need to be just over 1 inch.
To physically do this, use chalk and mark at least 3 stations on the rail head 15.5' apart.