The sky is not a good reference for middle tones. Its exposure value changes with time of the day, season, latitude and angle, amid other factors. For your 'rule of thumb', you aim the spot at the blue sky ~30° above the horizon. But again, you won't get the same exposure value in Florida than in Winnipeg. At my latitudes (~50° N), I don't have to compensate at all: the blue in the clear sky 30° above the horizon in the summer, in the late afternoon is middle blue.