How far is Neptune from Pluto?
Neptune is an inner planet to Pluto.
The average distances of Pluto and Neptune from the Sun are 5906.4 million (M) km and 4495.1 M km. (From NASA Planetary Fact Sheet).
Considering the mini/max distances during proximate alignments (conjunction and opposition) about the Sun, take the average. Then, the average distance in-between any planet and its inner planet is the average distance of the outer planet from the Sun, approximately.
Max = Outer aphelion + inner aphelion
Min = Outer perihelion - inner aphelion
The mean is mean distance of the outer planet from the Sun..