OK, I think it's from your own calculations, rather than was WT has printed/admitted - the 'Leavers' number is obtained by looking at the annual increase (or decrease) in publisher numbers compared to the number baptized over the same time
For example: Over a Service Year, if the number of publishers reporting increased by 100, and 110 were baptized, that means you are claiming there were 10 leavers
That's all folks!
DarK SpilveR