See the figure below. Write these down or store them in a spread sheet on the computer for each video frame. Point 1 is subtracted from point 2 in the x-direction. Divide each data point by the number of pixels in 1 m. Now the digitized points are calibrated to distance of the original action.
Velocity is calculated here as an average from one digitized frame to the next.
Therefore, the time related to this velocity is midway between the two digitized frames.