As mentioned earlier, during the development of your skill on the green, you become promoted from Lead to Second or Vice. While the initial effort has been on delivering the first bowls of the end, the Second or the Vice Skip maintain the head during the Skip’s delivery. The explanation of the Measure with both a string tape and the callipers covers the determination of score at the end of an end; but that leaves the ongoing determination of who is counting following each delivery by the opposing Skip. You are obliged to measure with your eyes.
Because you don’t usually have a lot of time during an actual match, practicing is best done with some supervision on an otherwise empty green. For this exercise, the supervisor will need a few sets of bowls of different colours and sizes, and perhaps 4 jacks. If you don’t yet own your own measuring tape with calipers, the supervisor should have them. This is not an exercise in measuring with these instruments; but an exercise in judging and being proven right.
The common errors are in comparing two different sizes of bowls; two or more bowls in different states of rest (on the running edge versus on the bias side); with the jack between two bowls rather than in a triangle formation; with one bowl (a toucher) in the ditch measured against the jack and other bowl not in the ditch; and with the jack also in the ditch. The organizer should carefully set up the scenarios and carefully pre-measure them, providing each participant with a score card. Each participant will make their decisions independently and note them on their card before the organizer performs the measurement to satisfy anyone who needs convincing.