I think I know what you are trying to do, but I don't think that you can get reliable numbers that way. For one thing, the different talent searches appear to have different threshold criteria. In other words, the population demographics as far as ability are not exactly the same. For example, I glance at that Duke chart another poster recently linked and was surprised that the highest Explore scores weren't as high as I would have expected based on stats from some other talent searches.
We have used three different talent searches over as many years and it may also be that the stats also shift over time. The criteria to participate may also change. I think at least one of the talent searches that we used (JHU CTY?) allowed entry based on state assessment in the advance category. In 3rd and 4th grade math, that would have included almost 2/3 of all the kids in those grades at DS/DD's school. For our first talent search in 2nd grade for my younger two kids, we did not have any of the listed tests but was still able to register based on parent recommendation.
I think some of these talent searches supposedly aim for the top 5% in reading or math, but in reality the net ends up being broader with state assessments and parent recommendations as just two possible explanations. I would not assume that the talent search kids are higher than top 10% although I would assume that they are mostly at least top 25%. So in your example with the 80 percentile, that would translate to top 2% to top 5% in a normal class.
