Together with other relevant government pamphlets (e.g. a "Memorandum of Instruction for Carrying out the Details of an Execution") the Home Office issued a Table of Drops to sheriffs and executioners.
There were basically three versions of it, with different degrees of compulsion. The table which was relevant for the period 1945ff was the one from October 1913. However, from 1939, it was customary to add 9 inches to the drop length taken from this table, and new executioner trainees were taught to do it by their instructor.
The drop tables can be found here.