MoN is absolutely right.
We basically just go with our gut and do what feels most tolerable. We never wanted DD to be operating below about the 75th percentile, though-- that felt unhealthy. So we've tried to walk the line between extracurriculars that range from 70th to about 99th percentile (with acceleration in place) and academics that are (ideally) about the 95th percentile.
The unfortunate reality is that most of the time, DD jumps back into the 99th percentile within a year or so from that 85th-95th immediately after acceleration.
Often you don't really get to choose a "best" fit, but one that minimizes problems-- or more likely, selects for particular problems and minimizes others.
Which problems are preferable is highly individual and depends on the child and the family.
My long way of saying... I'm not sure that anyone ever really knows where to accelerate to. Not really. You just try things and see if it works and feels okay.
