Leaving the lid partially on may not be enough on its own to minimize evaporation. I routinely leave the lid partially on my kettle to help reach and maintain a boil with larger volumes on my stovetop. I find that I actually get more boiloff with the lid partway on than fully off. I assume that this is because I can maintain a more vigorous boil. You will likely have to ease off on the throttle on your burner in conjunction with leaving the lid partially on to see a significant reduction in your evaporation rate.