"...Many online calculators assume a purity of less than 100% anyway..."
Actually every calc I've looked at (not an exhaustive search, but around a dozen) assumes 100% purity for NaOH and most of them are also 100% purity for KOH (if they do KOH calculations, that is).
But honestly, the NaOH that the manufacturer makes and the NaOH that the soaper actually uses are two different things. It is so very unlikely that NaOH stays at 99% purity under normal storage conditions. If your soap feels too superfatted, I would suspect your lye is far less pure than you think due to reaction with moisture and CO2 naturally in the air. Even storage in a tightly closed bag or container is not good enough, since NaOH degrades so easily.