A police inspector once gave me a bit of advice When stopped and asked if you have been drinking say NO they may believe you and say ok drive safetly and goodnight or they may make you take a breatherlyser it is 50/50. If you say YES but I have only had 1/2 of shandy they are legally bound...
The only definitive answer is dont have even 1 pint.
I can never understand why people go to a pub and have 1 pint then drink soft drinks all night or vice versa drink soft drinks all night and then have 1 pint before going home. If you are spending the time in a pub drinking coke or similar...