According to the link in your own question, a Pokemon's hidden power type depends only on whether its IVs are even or odd and nothing else, so a Pokemon with perfect IVs has the exact same hidden power type (specifically dark) as a Pokemon whose IVs are all 1. Additionally, there are only 16 hidden power types and 26 = 64 ways to have even or odd IVs, so different permutations of even or odd IVs result in the same hidden power type. For example, if your Pokemon has dragon hidden power, that means its IVs can be O/O/E/O/O/O, E/E/O/O/O/O, O/E/O/O/O/O, or E/O/O/O/O/O.
IV calculators are easier to use if your Pokemon has no EVs. EVs and IVs both affect stats, so if you know that your Pokemon has 0 EVs, you can more easily use its stats to narrow down the set of possible IVs. You can get a high level Pokemon with no EVs by using rare candies, the day care, or EV lowering berries.