I am wondering what is the right way of modifying that map. Lets say that oem is calibrated to 336NM and we are gonna change this x axis value to 350NM or so, what is the right way to get right amount of fuel for this value? Can i use linear interpolation?

from what i read it is indeed best solution.

im trying to understand this calculating. i have made somthing in excel. do you know how this goes?

Can you post your map so that i can use it for interpolation and put solution here so that you can check it?

For EDC16 I normally interpolate/make graph line equations for each RPM point between the last two torque values. I then change the X value in my graph equation giving me the new Y. This seems to work quite well but can sometimes exaggerate the 'spikes' put into the IQ conversion maps as these are obviously calibrated using real data.