I have a dozen 1 pound bottles that I collected over the years, and refilled them once so far, in one batch from a 20# tank. This batch will probably last me another year or so, since I only use about 2 per year to run a small stove. I checked the weight of each bottle before and after the fill to see how much was getting in, and the first 3-4 were overfilled. I cooled everything overnight on a -5F night.
I recommend watching a couple Youtube videos to see what's being done. Actually, it is not hard to overfill, and you probably don't want to do that; it will result in droplets of liquid propane sometimes getting through your regulator and causing flares. I ended up flaring off the excess with a torch, but have otherwise been happy with the results.