Gifted education should not be a "reward" for a certain test score. It should be speaking to a need that some students have for more depth, complexity, nuance, and so forth as well as a faster pace. And in order for the program to be ethically defensible, this need should *not* be present in students not identified for the program.
So instead of focusing initially on identification, you should decide what sort of program you can and want to offer. Then you design your identification plan so that it is able to select those students who will be able to benefit from the program.
As an aside, I have taught both gifted and non-gifted (though very bright) students algebra in a one-on-one setting, and one of the differences I notice is that the gifted ones need far less practice. This seems to be because the gifted ones see the underlying simplicity of whatever it is they are studying, whereas the non-gifted ones see it as a bunch of disconnected elements to be memorized. So, a 4th grader who is doing middle school math easily (so, being taught but not requiring an inordinate amount of practice) is more likely to be gifted than one who requires lots of after school time with rote practice to achieve the same thing.
