How far above the Earth's surface does interplanetary space begin? The Kármán line is often cited as the beginning of outer space.  The Voyager probes have identified the boundary between the heliosphere and interstellar space.  Is there a agreed upon definition of the boundary of interplanetary space?  Is it the Earth's magnetopause?
It it wherever is convenient for the article you are writing. There is no single consenus definintion of "interplanetary space".  If you are in orbit around a planet then you are not in interplanetary space, if you are between planets, then you are.  But there is no substantial change in the matter filling space as you move from planetary orbit to interplanetary trajectory

So you can use any reasonable point that fits the article you happen now to be writing