Find the longest shutter speed before stars start to trail, from your focal length and sensor. Choose the 300, 500 or 600 variant.
The 500 rule estimates the longest shutter before stars trail: seconds = rule / (focal length times crop factor). 500 is the common value; 300 is stricter for sharp pixel-level stars, 600 is more lenient. The NPF rule is more precise but needs aperture and pixel pitch.
It estimates the longest shutter speed before the Earth rotation turns stars into streaks: seconds = 500 / (focal length times crop factor). A 14mm lens on full frame gives about 35 seconds; the same lens on APS-C gives about 24 seconds.
They trade strictness for brightness. 500 is the common balance; 300 keeps stars sharper at the pixel level on high-resolution sensors; 600 lets in more light at the cost of slight trailing. Pick the one that matches how closely you pixel-peep.
Yes, the NPF rule is more accurate because it accounts for aperture and pixel pitch, but it needs more inputs. The 500 rule is the quick field estimate that gets you close.