Short answer, 5µs, which means the precision on a back-line to hog line split time is 0.01s*
Long answer is, it depends on your set of rocks... Chronocurl, does not work well with square rocks ;)
For the clubs we tested, circumferences are usually well within the margin of error of a half inch (leading to 0.05s BL-H difference). There is already a setting in the software to adjust for your average stone circumference.
Since the speed is determined by the width of the stone, a larger rock will appear to move slower. For each inch of circumference difference between two rocks (1/3.14 inch diameter is visibly bigger) you get about 0.1s difference on a 4.0 BL-H draw.
At Chronocurl, we understand that elite player need much higher precision. For that reason, we are finalizing features to allows customization of each rock diameter per sheet, per curling club in a way that users can load/share easily. We will maintain a database of rock diameter for each user and curling clubs that wish to share their information with us.
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*This uncertainty has been determined experimentally passing the laser beam through a Canon F100 shutter. Obviously, we cannot really tell if that inaccuracy comes from the shutter mechanism of the camera or the Chronocurl.
We cannot really tell what design criteria engineers at Canon had but a 5µs accuracy would lead to 4% error on the fastest speed of the F100 (1/8000 of a second (125µs)) therefore a 5µs uncertainty on the mechanism is pretty ok given the fact that modern (did I use that word) emulsions have tolerance of a full stop - 100%.
Bottom line, we're pretty convince the device is more accurate than 0.01s... But unless Canon wants to publish the accuracy of their shutter, it shall remain a mystery...