How do I correctly set the angle of my drop bars
Are there any tricks to setting the correct angle for the drop bars on my road bike, when adjusting my handlebars in the stem?
Is it just a matter of personal preference or is there a general consensus on what angle you should use?
Mostly personal preference. You may find that you prefer to have the angle slightly different depending on whether you ride mostly on the top, on the hoods, or on the drops, and you will likely discover that what's comfortable for a short ride is less so for a day-long outing. So adjust to suit your riding style.
I suppose I was really after how the mechanics do it when they set up all the new bikes in a bike shop. Sure many will adjust to suit themselves but a lot of people will leave it in the "generic" position. What is this position and is there a method/trick to setting it - like placing a spirit level across the first 90° angle after the stem?
Depends on the bar. So the hands feel comfortable on the hoods and also on the drops. Some bars have the two nearly parallel, some at a fairly large angle to each other, some bars have a shallow drop (which makes more angle on the drops comfortable), some have a deep drop (which makes a more horizontal drop position better).
Handlebars will be mounted parallel to the ground or angled slightly upward. While they may never be pointed down at all, they may be angled up slightly; allowed handlebar tilt is to be between 180 and 175 degrees with respect to the level road. The brake levers will preferably be mounted such that the end of the brake lever is even with the bottom of the bar. Modern bars, however, dictate that this may not always be possible, so tolerances are permitted within reason. Brake hoods should not approach anything near 45 degrees, as some riders with poor taste have been insisting on doing.
From the rules.