Here is my processing script in case you want to pursue this:
//"zip_code","latitude","longitude","city","state","county"
var data = [
["00501",40.922326,-72.637078,"Holtsville","NY","Suffolk"],
["00544",40.922326,-72.637078,"Holtsville","NY","Suffolk"],
...
["99929",56.449893,"-132.364407","Wrangell","AK","Wrangell Petersburg"],
["99950",55.542007,"-131.432682","Ketchikan","AK","Ketchikan Gateway"]
];
var result = _.chain(data)
.groupBy(function(item) {
return item[5] + ", " + item[4];
})
.map(function(data, county) {
var minzip = _.chain(data).pluck(0).map(function(zip) {return parseInt(zip, 10)}).min().value();
var maxzip = _.chain(data).pluck(0).map(function(zip) {return parseInt(zip, 10)}).max().value();
return ' ${minzip} <= $z and $z <= ${maxzip}, "${county}",';
})
.value();
var formula = '
var Number z = ToNumber([ZipCode]);
If(
${result.join("\n")}
)
';
console.log(formula);
