Calculating the absolute margin of error of a circuit
Convert ADC error counts (LSbs) to real numbers (voltages) and then add the equivalent DAC error voltages to the ADC error voltages. However, you also need to take account of the following for each and don't ignore any.


integral non linearity error
differential non linearity
gain span error
zero offset error
reference voltage error


All the above errors can be additive so, if you want worst case total errors, you must sum them for both parts. You should also consider temperature variations from ambient and how much this makes things worse. For instance, the voltage reference is likely affected most and this problem can be very significant.