Actually, the sunsigns are from traditional astrology and what you're interested in is cosmobiology.
there are 3 areas in the 90 degree wheel. First, cardinal where your mercury and neptune fall, and next fixed, where your Jupiter is placed.There is no midpoint between these planets but there may be for others.
For example, a planet at 26 degrees in a fixed sign such as taurus, leo, scorpio, or aquarius would midpoint your Mercury Neptune combo
here's a good book on midpoints in astrology (cosmobiology)
regards - Johnny