A method of procedure based on experience and common sense.
Do you know what a 'ruler' (尺子) is? And a thumb (拇指I)?
Literally (word by word) it means you dont work too exactly, but exact enough. You use a long and precise millimeter scale (尺度?) for your job, but you don't measure by millimeters. Two or three centimeters more or less don't matter. You know, when it's enough. You don't have to look.
I think Alex's second example is the better one.