6.1.1. Steep hills are signed using diagrams 523.1 (S2‑2‑18, see Figure 6-1) and 524.1 (S2‑2‑19, see Figure 6-2) together with associated plates. The gradient is calculated using the tangent of the angle concerned, although in practice it makes little difference whether the sine or the tangent is used. The gradient on signs must be expressed as a percentage; old signs showing a ratio may remain in place until life‑expired. 6.1.2. The sign to diagram 523.1 should normally be used only where the gradient is 10% or more. The actual gradient to the nearest whole number should be indicated on the sign, e.g. a gradient of 10.4% should be signed as 10%, but 10.6% as 11%.