We do not consider our variable transformers as devices that require calibration unless they include a digital or analog meter. A variable transformer only provides an output that is a ratio to the input. This is accomplished using transformer action (turns ratios). There really is not anything that can change this ratio. Also since it is a ratio of the input voltage the output will change with input voltage changes. External or optional internal metering should always be used if a precise output is required.
Instead of a calibration if would be more beneficial to clean and inspect periodically. Cleaning the brush track and inspecting the brush for wear could possibly increase the life of the transformer. The brush could possibly changed prior to failure. Brush failure often damages the coil making the transformer beyond repair.