Polar graph paper, 30 degree spokes
Concentric rings with a spoke every 30 degrees, which divides the circle into twelve.
Opens with these settings already loaded. Change anything you like, then download as PDF, PNG or SVG.
Twelve divisions suit clock and compass work
Thirty degrees gives twelve sectors, which maps directly onto anything already divided into twelve: hours on a clock face, months around a year, or the twelve-point compass used in some navigation and surveying work. It also happens to be coarse enough to plot by eye without a protractor, which the finer 15 and 10 degree spacings are not.
Thirty degrees is a common angle in its own right
Plenty of geometry and trigonometry work lands on multiples of 30, since 30, 60 and 90 are the angles of the standard set square and the ones whose sines and cosines students learn as exact values. Having spokes at exactly those angles means you can construct and check by eye rather than measuring, which is the practical argument for choosing the division to match the problem.
Change it when the data changes
Spoke spacing should follow what you are plotting, not habit. Fifteen degrees gives 24 divisions and suits general hand plotting. Forty-five gives eight, which suits a wind rose or an eight-point compass. Sixty gives six, which is what the radar chart sheet uses. All of it is adjustable in the generator.
Printing it accurately
This only measures correctly if your printer doesn't rescale the page. In the print dialog set Scale: 100%, not "Fit to page" or "Shrink oversized pages". If you're not sure it worked, print the calibration page, measure the ruler on it with a real ruler, and it will tell you exactly what your printer is doing to the page.