Ops
There was no hard and fast rule laid down and a huge amount depended upon the C.O. of the Squadron. There were some constants and amongst them were not carrying on for a reason that could not be well and truly explained and believed. This would never count as an Op wherever you turned back from. If this happened with the same individual more than was deemed normal there were some very hard examinations made with questions asked to the crew members and drastic action would be quickly taken. It also differed fromn Command to Command and even from Squadron to Squadron. There was also a vast difference in what constituded an Op. Mine laying, for example ,invariably counted as a "half op" however close to the enemy coast you became. Later, much later in the war when the attacks on such targets as V1, V2 sites , ground troops, general bombing of so called tactical targets became more current, the whole system changed dramatically and the duration of a tour became more the decision of the immediate Officer commanding the Squadron.
Apropos the entries in Log books I never encountered any rules whatsoever as to how entries should be made and , apart from the obligatory monthly summary on hours flown and dates, the log books were never examined but had to be signed at those times by the Flight Commander. However it was customary but not mandatory that night flying would be inserted in red ink but only in the night flying column. I, like a lot of people but not everyone, entered my Ops in red ink in the remarks column and underlined the whole entry with red ink but this was a personal choice and by no means everyone's method.
One last bit of friendly advice. Ask these questions now ! Regle