Thank you very much, I will try it tomorrow, have a nice day.Yes, correct - but if you have an auto-ranging digital multimeter it doesn't matter (in this case), which way round the minigrabbers are connected. You just need the reading to be 20mV and it doesn't matter if the meter says -20mV or +20mV after adjustment.
If you have an analog multimeter (with a scale and a pointer), then polarity is important as the meter wont read properly with the minigrabbers attached with incorrect (opposite) polarity.