There were several articles in early issues of Measurement News showing how to do such an offset using triangulation w/ a tape measure. It's good to use in a pinch.
But for a 300m cal course that is going to be used many times for measurements, I wouldn't feel confident without a steel-tape or laser layout.
You can avoid or lessen this problem by locating a course on a "No Parking" street. My own "home" course seldom has parked cars on it (all the houses have driveways) but every so often there will be a visitor or landscapers parked on it. This happened more frequently on one side than the other so when the street was repaved I remeasured on the quieter side of the street.