Im lazy, I just do the "Blue Rhino" exchange.When I said "full fill" I meant the 80%/20 pound mark. The pre-fills just seem to have a tendency to only fill to eighteen pounds or less. Over a season that can account for a few extra tanks. When we first started using the Big Buddy I tracked how many hours of use I got out of a tank and the pre-fills always came up short by a few hours compared to the "professionally" filled ones.
There's a difference, too, between vendors. I tried another place, at first, and had wildly varying results from tank to tank as in up to four hours of difference and that was with the same guy "filling" those tanks. It sure didn't give me a warm, fuzzy, feeling.
But then I only have three tanks, and mostly just use propane to grill a steak once a week. The two spares are in case I need emergency heat. I have a natural gas stove, natural gas water heater, and a wood (or gas insert) burning fireplace, so I dont need to use the Tank Top Heaters very often. Usually just for some foolish outside social event in the winter.