Ladder Setback and Angle (4-to-1 Rule)

Set the feet one unit out for every four units of height. That is the 4-to-1 rule, and it puts the ladder at $\arctan 4 = 75.96°$, close enough to 75.5° that OSHA, CSA and the ladder's own label all quote roughly the same figure. A 16 ft contact height wants the feet 4 ft from the wall. Too steep and the ladder tips backward as you climb; too shallow and the feet skate out from under you, which is the more common accident and the more serious one.

The field check needs no tape measure. Stand with your toes at the ladder feet and put your arms straight out at shoulder height: your palms should just reach the rung in front of you. That is the "firefighter's check", and it lands within a couple of degrees of 75° for almost anyone. Many extension ladders now carry a pitch indicator on the side rail that does the same job.

Two rules ride along with the angle. An access ladder must extend about three feet, or three rungs, above the landing so you have something to hold while stepping off, and that extension is not part of the height in this calculation, which uses the height of the contact point. And the base needs firm level footing: the geometry is exact, and it means nothing at all on soft ground or an unsecured ladder.