Just out of curiosity, I decided to try this out a few days ago. The street in front of my house has a concrete curb that extends out onto the asphalt road surface for about 15 inches. This provides a nice straight line to follow. I put two pieces of tape on the road, approximately 25 meters apart.
I practiced the riding technique one time before trying it for real for 3 trials. My wheel is marked in 100ths of a revolution, so it's possible for me to see very small differences in two successive trials. I found that there was no more than 0.002 to 0.003 revolutions difference in the readings of my three trials. For a 25 meter course, this corresponds to variation of about 0.02%, which is a good bit smaller than variations I see when I ride 100 to 300 meter cal courses.
I did this quick test just to check how repeatable rides like this are. I will check later how it compares to cal constants I get with longer rides. But it is easy for anybody to do this same quick test, even if you don't have your wheel marked up. Just put two pieces of tape on the road about 25 meters apart. Then mark your wheel where it rolls over each piece of tape. If you repeat the rides starting at the first mark you can see how close you get to the second.