What percentage reflectivity would be necessary to keep the average temperature of a planet exactly at the freezing point?

That would depend on the intensity of the incoming solar radiation ... essentially

the planet's distance from the sun ... as well as on the size of the planet, which

determines its total absorption area.

Mercury doesn't have to absorb sunshine as effectively as Pluto does, in order to

maintain any given surface temperature.

(You didn't say the "freezing point" of what ?)