To determine the phase between two planetary bodies, use the slowest one as the starting point. The slowest one takes longer to travel through all the zodiac signs, than the other. For example, it takes Pluto approximately 248 years to travel from Aries to Pisces, but only 28 days for the Moon to do the same.
If you want the phase relationship between the Pluto and the Moon, enter the sign and degree of Pluto in the Slower Moving Body field. Use the Moon’s sign and degree in the Faster Moving Body field.
Scroll down this page to see the speeds of the planets.