Breathalyser limit for a club

Hey, so a bit of background I work in a nightclub in London and one of my job is to stop people who are too intoxicated entering to protect our licence, but this weekend I had several discussions with people who were too intoxicated about being too far gone and they were telling me to breathalyse them, so I went and bought some but can’t decide on a limit which is acceptable because I understand people obviously pre drink so it will come up more than the legal limit but whats too much?