I don't think that's accurate. Considering that honey greatly differs in sugar-water ratio, it is safe to say that either going by weight or fluid amount will be fairly inaccurate, unless you already know your OG from a different batch, and are sure you are using the exact same honey from the same apiary. It's surely possible it would of reached that high in such a short amount of time, but high tolerance yeast who's tolerance is 18% usually start to lag a little at the last 3%, where yours has not. I'm not sure what you mean by "Adjust that gravity reading". Please specify.