How do I reduce planetary photometry to standard viewing geometry using phase angle? I'm trying to make light curves for Pluto using old data. Most papers give magnitudes "reduced to standard viewing geometry" of r = 39.5 AU, &Delta; = 38.5 AU, and &alpha; = 1 degree, where r is heliocentric distance, &Delta; is geocentric distance, and &alpha; is phase angle. 

When papers give apparent magnitudes, I'm left to perform the reductions myself. I'm fine with the adjustment for r and &Delta;, but how do I reduce to phase angle of 1 degree?

I would also really appreciate a good source if you have it on hand.
Phase functions are nonlinear in general, but Pluto presents such a small range of phases to Earth that a linear approximation will do:
about 0.03 mag/deg for Pluto and 0.08 mag/deg for Charon, or 0.04 mag/deg for the two together.
The Stern and Tholen book summarizes the estimates up to 1997. 
A search for Pluto phase coefficient
yields some of the papers behind those and more recent estimates.