If the index defect is a depression, bridge it, add a ruler or tape perpendicular to the ‘bridge’, measure down, record and image. 40 mm is the rule of thumb. Same goes for any differential in level, lateral or transverse to the highway.
If the defect is standing proud of the surface, balance a spirit level across and support one end when the bubble is dead centre; then measure down from the other and record.
In the spilt concrete case of Thomas v Warwickshire CC the Court decided a 20 mm protruberance was sufficiently dangerous to have merited the Council’s attention.