Well, the only reason to have an "error" is because you don't know the actual value (which is what I think you mean by the "accurate mid-value". If you know it, then, sure, use it. But most of the time you don't so you must an estimated value. In that case, all you can do is estimate the error, anyway.There is often a difference of 0.05% or so between these methods. I don't know which method to use.