Actually, you CAN calibrate a loaded field voltmeter with a regular voltmeter. Connect the loaded voltmeter to the battery pack and then connect the unloaded meter in parallel. The battery pack will still be loaded and you can then compare readings. Adjust the "hobby" voltmeter to match the "good" one. You'll now have the loaded voltage reading at the point where the two voltmeters came together. Usually, you do this at the battery connector.