How do I reduce planetary photometry to standard viewing geometry using phase angle?
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.