Set the scale by measuring something on the sheet whose length you already know.
Calibrating sets the scale from the drawing itself: you measure something whose real length you
already know, and the scale is worked out from what you measured.
Use it when the sheet does not say what scale it is at, when it has been printed or scanned at
something other than its stated scale, or when the stated scale does not check out against a known
dimension.
How
Start from the scale editor. The Calibrate button sits next to the equation you would otherwise type.
Enter the length first. A dialog asks for the real-world length of what you are about to measure. This comes before the measuring, not after it.
Click the two ends. The scale is computed from those two clicks and the length you entered, and the settings window comes back with it filled in.
Picking something to measure
The measurement is only as good as the thing you measure, and the error scales with it:
Longer is better. A few pixels of imprecision over a 100-foot dimension is nothing; the same slip over a 2-foot detail is a scale several percent out across the whole sheet.
Prefer a dimension string over a drawn object: it states what it is supposed to be rather than what it happens to be.
Avoid anything that may not be to scale, such as a detail bubble, a note, or a graphic dropped onto the sheet rather than drawn on it.
A graphic scale bar is a good target. Measure the full bar rather than its first division.
Notes
Calibration needs a valid value in the scale unit field before it can start. The field accepts fractions, which is how architectural scales are written. An entry it cannot read stops the calibration with a warning rather than part way through.