How to calculate diesel fuel economy from OBD11 codes

Question

For a Hyundai i30cw manual there is no fuel consumption meter available. I have a generic ELM327 USB PC interface for which the accompanying software reads a limited number of codes including the apparently relevant parameters of speed, revs, inlet manifold pressure and temperature, mass airflow, power %, exhaust gas recirculation  and others. I have tried with a spreadsheet to calculate something sensible from these parameters but the results appear quite inconsistent.

What is the right formula to calculate fuel economy for a CRDI motor?