From the diagrams I saw, it should be linear. The largest share of the return fuel is from the feed pump, but what gets forced through the injectors is controlled by the position of the control sleeve (controlled by the throttle) and the plunger moving back and fourth.
One could describe the control lever position to control sleeve with a function, or make a good guess at the demands at idle and at full throttle and come up with a formula that way. Alternately one could pop off an injector and measure how much fluid comes out of the injector after X revolutions for different throttle positions and map it that way. You might have to add a tps and an rpm signal though.
http://ecomodder.com/forum/showthrea...html#post63034
