Oil central heating

[Whickerman]

We have (for several years now) been using diesel in our home heating system. No problem with that. But I've had the system checked and the oil pump seems to be set for Kerosene (the pressure is set at 12 bar vs the 8 bar it should be for diesel). Again, no hassle, I've adjusted it and it's working perfectly, as expected.
My question is this. Roughly what % of fuel have I wasted by having too high a pressure set?