Ask the spectator to indicate a 4 X 4 square by making lines around 16 dates. You then write down your prediction and hide the number (see below for how to produce your prediction). Have them circle any date and to then cross out all of the dates appearing in the same column and row. Then, they are to circle any remaining date and also cross out the remaining numbers in the same column and row. After this procedure is done two more times there will be four numbers that are circled. Ask them to add up those four numbers and give you the total. You then reveal your prediction corresponds perfectly!
To calculate your prediction, find the sum of either of the diagonal corners and double that number. Above, this would be 2 + 26 (or 5 + 23) which doubled would equal to 56. As you can see, this is the sum of the “freely” chosen four numbers.