An offset is the most common bend an electrician makes, and the one most likely to go wrong on the first try. Two marks, two bends, and a length of conduit that either clears the obstruction or does not.
The multiplier sets the distance between marks
Multiply the depth of the offset by the multiplier for the angle you are bending, and that gives the distance between your two marks. The common ones are 6 for 10 degrees, 2.6 for 22 and a half, 2 for 30, 1.4 for 45 and 1.2 for 60.
Shrink is the part people forget
Bending an offset shortens the run, because the pipe now travels sideways as well as forwards. That loss is the shrink, and it grows with the angle: roughly a sixteenth of an inch per inch of offset at 10 degrees, three sixteenths at 22 and a half, a quarter at 30, three eighths at 45 and a half at 60.
A worked example
You need a 6 inch offset at 30 degrees. Multiply 6 by 2 and your marks go 12 inches apart. The shrink is 6 times a quarter, which is an inch and a half, so add that to your measurement before you place the first mark.
Choosing the angle
Shallow angles give a gentler offset that is easier to pull wire through but uses more length. Steep angles save length but make the pull harder. Thirty degrees is the usual compromise, and its maths is the easiest to do in your head.
BendPro is an app that gives you the distance between marks, the shrink and a diagram of the bend once you type the depth and pick the angle. It works in inches with fractions or in millimetres, with take-up for four common benders built in.