Depends a lot on horse an fence but a good start point is 3 strides away then move depending on how horse manages. I prefer to use a plank or square pole in this situation as it wont roll if horse steps on it.
Depends on the size of the fence and size of the horse. For anything 14.2hh and under, I would opt for 2 and a half strides from fence. 3 strides for a larger horse.