If you're trying to squeeze every last drop by taking one engine out of the equation, then having that engine at idle is pointless, burning fuel for no torque, so shut it off completely.
Best range configuration from the RFM is the best place to start, but if you have groundspeed and fuel flow data available, then work the actual numbers. Quantity of fuel burnt per mile, whatever altitude/power/wind/Nr/OEI combination gives the least has to give the most range.