You might like this.
It is from the UK ACPO Manual of Guidance on the Police Use of Firearms
11. TEMPORARY UNFITNESS TO BE ISSUED WITH A FIREARM
11.1 The consumption of alcohol or the use of drugs in medical treatment or otherwisemay reduce the alertness and performance of an officer. Officers carrying firearms
should not consume, or be unfit whilst under the influence of, alcohol. Any officer who is receiving a course of drug treatment should seek the advice of their own doctor or Occupational Health Unit concerning the possible side effects. Any other concerns, or medical advice given to an officer should be brought to the attention of their immediate supervisor, who should take appropriate action.
There used to be a 24 hour before rule. Now, however, an officer's fitness to carry, given regard to all circumstances, is considered by the individual and their supervisor.
Just because you had a drink eight hours ago and there is residual alcohol in your system, doesn't necessarily make you unfit to carry out a task.
This all goes back to what is a safe level.
Having been fully trained as a custody suite breathalyser operator, you would be surprised how much you do have to drink to get it to provide a big reading. One of the main things is how much food you have eaten. Half a lager on anempty stomach is a recipie for disaster