Calibrating a tube tester?

[Kamakiri]

For testing my tubes, I've got an old Seco drug store type tester. Always seemed to work pretty well, but I'd like to calibrate it, as there are a few readings I've looked at recently, but don't trust. There are controls underneath to do so.

Is there any secret formula for doing so, other than checking it against another tube tester and matching the emissions readings? I've heard mention of a calibration tube, but haven't seen anything for sale.