I have also tried to calibrate but believe that there is a bug. The way I understand it is that you must calibrate the different vertical scales of 50mv,0.1v, 0.2V, 0.5V, 1V, 2V, 5V, 10V. First I connect the probe to ground and then adjust to read 0.00. Then I connect the probe to the input voltage, for instance 250mV for the 50mv vertical scale which is the first row. I then ignore diff, it looks as if it is correlated to the “Votage” column. In the “Votage” column I then adjust until the Voltage that is displayed is the same as on my multimeter. The problem is that there seems to be a bug. I can adjust the voltage lower, but not higher, it sometimes works and goes higher, but other times it will go lower, but not higher.
The interface is not very good because if you then go back to the beginning of the first cell in the row the previous value is overwritten, so you have to be careful and move to the next row.
This is how I think it works, but I might be wrong.