It sounds as if you might have got confused by the inclusion of the 'armhole' phrase. You need to measure the whole front - ie. from the bottom hem up to your needle. Unfortunately it sounds as if you have gone too far but try to think of it as good practice of the stitch for the future bits that you are going to do
Where the armhole comes in is when you are working out whether to stop on a right-side row or a wrong-side row. If you stop on the wrong side, you can end up with your pattern being 'back-to-front' - with stocking stitch on the wrong side, for example. The 'stopping at armhole edge' means that when you get to 19.75 inches, you should stop on a row that ends at the armhole side rather than at the neck side.
Hope that helps!