When considering a grade skip, where is the optimal "place" for a child to be in a class achievement-wise? Where would the child feel challenged but not overwhelmed?

So, for example, say that a child scores at the 99th percentile overall when compared to her age peers. When compared to peers one grade level up, she places at the 98th percentile, two grade levels up, at the 85th percentile (I'm making these numbers up), three grade levels up at the 50th percentile, and so on.

It's pretty obvious that one grade level up is going to be too easy. But is it better to be at the 85th percentile or the 50th?

Let's assume for this discussion that social and developmental concerns are being ignored and that the distribution of achievement in the classes mirrors the national distribution.

Is there some common wisdom about this?

Thanks!