Agree with Morgy ..
I thing the fuel consumption curve (vs trimmer position) would be an inverted parabolic, so you should do a few test with your own engine and canopy combination to get the best trimmer position ...
No need to fly hours, just set the desired trim position, adjust the throttle to make it at cruising speed (not climb or sink) and write down the speed vs RPM. repeat for few other trimmer position. Assuming the RPM is linier to fuel consumption (would be better if we already have data for RPM vs fuel burn rate), we can calculate and draw the fuel consumption curve and pick the best trimmer position to apply ...