Each pair of stars should straddle closely to the meteor's path. Repeat for the other pair. After some practice we find that this comes naturally, and it gives accurate results. The magnitude comes from comparison with field stars, though after a while it is possible to judge the brightnesses of most meteors directly. A being the slowest equivalent to about 2 degrees per second, and F is the fastest corresponding to 25 or more degrees per second.
Numerical estimates are too difficult given the magnification.
The velocities are needed in the radiant analysis. OO means it traversed the whole field. 10 means that the meteor started within the field, but moved outside. If there is a persistent train we estimate its duration and occasionally make sketches of its decay. Depending on the weather conditions and alertness of the observer, we take longer and more-frequent breaks than visual observers, as telescopic observing does require more concentration, especially when the rate is low.