Was right for my son and 3 of my friends.
On accuracy, I found this "When using the calendar to plan your baby's gender, to maximize the chance for correct prediction you should avoid conception during the first or last ten days of a gender period (period for conceiving a certain gender according to the calendar). The reason is that there are unknown, but apparently gradual changes taking place between two successive gender periods. During these changes the selection of gender happens more or less by chance.
Bevzibubble, I guess this might be why??