Expanding on his work, I've come up with a methodology to use one of these instruments to measure a 300m cal course very quickly, and I think very robustly.
One of the shortcomings of these consumer-level devices is that their stated maximum range is less than 300m. Therefore, it will be necessary to take two measurements in order to measure the necessary 300m. The strategy I came up with to do this is shown in the photo below, where I place two targets, one at either end of the cal course, and place the EDM somewhere close to the middle between them.
My targets are 18-inch square section of particle board with a smooth white finish. These are supported by a 4x4 with a groove cut out.
The EDM is mounted on an equatorial mount similar to Mike's, which allows for fine adjustment in order to point the laser beam at the target. This is a necessity at a distance of 150m. The measurement is relative to the rear of the EDM so I use a square to place a piece of tape on the ground at this point.
I then loosen the mount and spin it around 180 degrees to point at the other target, as shown in the video link below.
http://www.dukerdog.com/USATF/EDM/Spin180.mp4
Then I take the measurement to the other target, and again place a piece of tape at a location at the back of the EDM.
Finally, I measure the distance between the two pieces of tape. This distance will be subtracted from the final measurement.
In reality, this 180-spin offset measurement can be measured ahead of time, and doesn't need to be measured onsite. So the entire procedure consists of placing the two targets at the endpoints. Taking one measurement to one target, spinning the EDM around, and taking the second measurement to the second target.
I tested this procedure out on a friend's cal course he measured in front of his house. This course is 1000 feet, or 304.8 meters.
EDM measurement to south target: 151.128m
EDM measurement to north target: 153.866m
180-spin offset: 0.230m
Total course distance = 151.128+153.866-0.230
= 304.764m
So the EDM measurement was 36mm shorter than the stated distance.