It really depends on the box itself, a good box can be 80%+ 10s whereas a worse box can be only 9s due to bad centering, dull cutting blades, stains, etc…all these factors differ from set to set and even from print run to print run. You would have to have a massive sample size in order to correctly estimate the odds.
@mojotbone, dude those results freaking rule! What an exceptional box opening and grading! 3 extra and a boatload of 10s, amazing! What went wrong with the raichu?