You would normally specify calibration limits in terms of +- X%.
The problem with a zero point is overcome by starting your calibration at 10%. A general rule of thumb is to calibrate at 10, 25, 50, 75, 90 and 100%.
Remember also that as well as the actual tolerance discussed above, you must also allow for uncertainty in measurement, i.e. the accuracy and least significant bit of the calibrating instrument.
Calibration Spec
