The analog output accuracy is about 1.8mV plus 0.083% of reading. When set to 5.0 volts, the accuracy is 1.8mV + 0.9mV. When set to zero, if your multimeter reads higher than 1.8mV and is less than a year old, I can arrange for free factory calibration. Otherwise, the USB-1808X fee is about $150.00. What is the make and model of your multimeter - I would like to check its accuracy too.
As for calibrating the device yourself, Measurement Computing chose not to add calibration functions to the programming interface. Instead, you could do something simple like check the offset and adjust each setting by that amount in your code.
Regarding the Python example, it is normal for the analog output to remain set to the last value. If it is a problem, add the code to put it to zero before the program exits.
