I guess the principle is the same as for helicopters:
Index = [Mass(Arm - RRA)]/x where arm=CG position, RRA=the CG position of the main rotor reference axis (equivalent to say 20%MAC in FW terms) and x=a convenient number
For DOI calculation, take the dry op mass of the aircraft (ie with all role equipment and crew) and the calculated dry op CG position (the arm). Correct the arm for the RRA (to get the + or - value). Divide the whole sum by 100 (in metric) or 10000 (in inches) to get a basic index value. For convenience we then add a random number (30 for most types) purely to have a positive index value.
Example for a Super Puma L2:
DOM say 6000 kg
CG say 4.66 metres aft of datum
RRA is 4.67 metres aft of datum
DOI is {[6000(4.66-4.67)]/100}+30 = 29.4