Smart! with this set up you could slide the disto to the center (over the head) measure this distance then slide the disto out to one end and just set the same distance, done!
Indeed. In practice I think the fastest way will be:
1. Center the Disto over the center of the ballhead or geared head.
2. Rotate to find the shortest distance. You can do both yaw and pitch at this time, but the pitch requires calibrating the pitch rotation axis, like in a pano head, if the highest accuracy is required. At a large distance a slight off pitch rotation axis wil become less important.
3. Slide the Disto to the left on the bar, and shift the bar to the left in the head's clamp, and look for differences in the distance. Rotate the yaw angle to compensate, and check by shifting the Disto to the centered head position again (now at the right of the bar).
4. For additional verification, shift the Disto to the right of the bar, and the bar to the right in the head's clamp. This will double the measuring base offered by the bar's length.
5. Hope that nobody bumps into the tripod ...