When using beersmith, I have set up my system numbers (still rough) and it helps me to know what volumes to use. I almost always get exactly what I need into the kettle this way, and usually have a little left that doesn't get boiled. The answer to your question is that it depends. It depends on the gravity of those final runnings, it depends on how short you were. It depends on your boil off rate. The first 2 answers kind of answer your question, but your shortage ay only be a result of a higher boil off rate than expected or inputted into the online calculator. I am still trying to dial in my system, i almost always have the right amount into my BK, but my boil off varies due the weather changes here: 30-50° for winter time brewing like this past Sunday, and up to 100° for Summertime brewing. Every time I think I have it dialed in, it looks off to me during the boil and i try to adjust on the fly when I should just leave it alone and see what results I get. Hope this helps a little. If you were short by 1/2 gallon, those extra runnings might have gotten you your expected volume, but then your gravity would likely have been off. For your first all grain batch, or your next, it is up to you which is more important: the volume you want or the gravity you want, at least until you have solved all the variables. Someone with more AG experience than me(1 year) will add more I am sure, and am interested in hearing