I've heard a lot of benchmarks over the years, but in our experience, the "best" placement is the one that allows your child to easily reach 80% in the coursework, and one in which s/he would have to work for the top 10% of performers in the class.
I wouldn't know what that looks like, however... we've always accelerated DD into material that was EASILY within her reach.
As Dottie says, they seem to cruise at top gear for a while, happily closing any gaps that may have existed prior to the acceleration. It's often more useful to look at what they are doing outside of school for clues about whether or not they could manage the higher level material.
The Iowa Acceleration Manual is a good guide to what "appropriate" looks like for a given child-- the OP can offer more advice there about specifics, as I've not seen it in person.
