...I might be confusing myself now
I always assumed that it had to be 300nm "straight line" distance, but I'm prepared to accept that I might be wrong about that
Actually, just found an FAA reference here:
FAR 61.129(4)(i) states for a single engine airplane is:
One cross country flight of not less than 300 nm total distance with landings at a minimum of 3 points, one of which must be 250nm from original departure point..