Steve, this being not my arena I cannot answer precisely right now, my understanding though was that the reference is the plane of the ecliptic, following the table by Berg illustrated in this link. So the boundaries are defined by astronomical reference.
https://en.wikipedia.org/wiki/Sidereal_and_tropical_astrologyFor the purpose of determining the constellations in contact with the ecliptic, the constellation boundaries as defined by the International Astronomical Union in 1930 are used. For example, the Sun enters the IAU boundary of Aries on April 19 at the lower right corner, a position that is still rather closer to the "body" of Pisces than of Aries. The IAU defined the constellation boundaries without consideration of astrological purposes.
