There are no hard and fast rules about charges. In my limited experience, I could tell when the elephant were serious.

It is best if you can wait until they are close, maybe five yards. Though the brain shot is nice, a good frontal heart shot will probaly stop them too.

Hitting the brain on a frontal shot on elephant is tougher to get done than imagined, especially in a charge. When they are close, their head is down and the tendency/instinct is to shoot too low.

One can Monday-morning-quarterback charges after the fact, but I would guess that without tons of experience in charges it is mostly a crap shoot. Sometimes you get lucky. I used to dream about it all the time. Maybe that helps!