The document and the formula are correct.
When measuring the angle of the propeller blade, this is taken, as the document says at the 75% blade radius station. The blade radius station is denoted as 0% at the blade hub (an impracticality) to 100% at the blade tip. Remembering that each blade station describes a helix as the blade rotates, the blade pitch angle is larger near the hub and smallest at the tip. The convention is the blade angle for calculating geometric pitch is taken at the 75% blade radius station.
The geometric pitch is a measure of how far (in inches) the propeller would advance with no slip, thus it uses 0.75 (the blade radius station) x diameter to calculate the circumference and multiplies this by tan of the angle.