Mercator Scale

There are two schools - at least - to plot a line of position with enough details.
Mercator templates are one, plotting sheets are another one.
The plotting sheets rely on the Mercator scale, available at the bottom right corner of the document above. It gives graphically the ratio between one degree of latitude and one degree of longitude.
The Mercator projection relies on the concept of "Increasing Latitude", that gives the value of the latitude, in degrees of longitude...
The increasing latitude λ, for a latitude φ is given by the following formula:

λ(φ) = ln(tan(π/4 + φ/2))

The Mercator scale gives the the ratio between one degree of latitude and one degree of increasing latitude at the given latitude. In consequence, its formula is

ratio = (φ - (φ - 1)) / (λ(φ) - λ(φ - 1))

simplified as

ratio = 1 / (λ(φ) - λ(φ - 1))

also written

ratio = (λ(φ) - λ(φ - 1))^-1

That's it!
The main difference I can see between the two methods is that the Mercator templates can be reused, just like a chart, and the plotting sheets are just a draft, to be dropped after used. That's just my opinion. Whoever has a better - or other - one is welcome to speak up.

Mercator Templates

Using the ChartComponents package, I generated some Mercator Templates (aka Mercator Plotting Sheets) that can be used for celestial navigation. They're available as pdf documents in different sizes: 11" x 8½", 11" x 17" and 17" x 22".
The Letter format (11" x 8.5"), which is quite small. I need to try it to make sure this is not too small. I reduced on all versions the latitude span to 3 degrees (instead of 6, on the charts published by the French SHOM, format ½ Grand Aigle), for the definition of a mile not to be too small.