Thought of a great example of what I mean.
I have a Kangaskhan who is EV'd in Speed. His Speed IV is 29. Now, if I just put the stats into the calculator with no EV points, its IV comes up as 31. This isn't accurate as I know it's EV'd in that stat. So, once I put in the right EV's and recalculate it, its Speed IV falls back down to 29.
This is what I've been meaning. Without knowing its EVs, the best thing you can do is guess but won't have accurate IVs.