I measured a course this morning. Rode 2 towns away to meet the race organizers near a cal course I'd laid out. Calibrated, did the 2 rides, then realized there was another cal course right near the finish of the race course, and by using that I could head directly home instead of stopping halfway to re-calibrate.
Did the first ride and instantly realized something was fishy. My pre-cal constant was 142.85 (using the Protegé method). Post-cal was 145.10. Then I realized the post-cal course was 1,000 ft., while the pre-cal was 300m.
This second course was originally laid out by another measurer in the state who thinks Imperially rather than metrically. When it was paved over, I laid out a 300m course in its place. The road was repaved AGAIN and this time the other measurer laid out a 1,000 ft. course.
The lesson is, don't assume a cal course is 300m (or 1,000 ft. for that matter).