Basically the rule is that for every 60 miles travelled, 1 mile off track is equal to a track error of 1 degree. (eg if you have departed from point A and your planned track is 360 degrees, and at point B which is 60 miles from dep. you find yourself 1 mile right of track, your actual track has been 001 degrees.)

The 60 to 1 rule (often written as the 1 in 60 rule) is a fundamental mathematical "rule of thumb" used by pilots for navigation and flight path calculations. It states that for every 60 units of distance traveled, an error of 1 unit in track results in being 1 degree off-course. For example, if a pilot is 1 nautical mile (NM) off-course after flying 60 NM, they are exactly 1 degree off their intended heading. This rule is highly versatile: it is used to calculate drift correction, descent gradients, and intercept angles. If a pilot needs to descend 3,000 feet in 10 miles, they can use a variation of the rule to determine that a 3-degree flight path angle is required (since 3 degrees at 60 miles is 3 miles, or roughly 18,000 feet). While modern GPS and flight computers handle these calculations today, the 60 to 1 rule remains a vital mental skill for pilots to quickly cross-check their instruments and maintain situational awareness without relying solely on electronics.