The lower the curve, the higher the annualized efficiency will be, but if you go too low the temps might not keep up. Experiment to see how low you can get it. At 3.7 it's obviously super-high, to the point where you under/over shoot the setpoint temp like crazy. It's like trying to ease through a parking lot in a custom hot-rod- it's either stalled cold or screaming out ahead, with not enough middle-speed range to run nice and steady at the speed you'd like.