One metre out for every four metres up, measured rather than judged
The angle
Base one metre out for every four metres of height. Stand at the base with toes to the feet and arms straight out; palms should just reach a rung.
The top
At least a metre above the landing point if you are stepping off. Tie it, or have somebody foot it, every single time.
The rule people break
Three points of contact, always. Carrying anything in one hand means one point is a hand holding an object rather than the ladder.
