Finally got around to measure the amount of water in the water change pail. It's an old Reef Crystal container. It's 5 gallons. I thought it was more (7.5-10 gallons). So I've been reporting a higher water change percentage than it actually is. IMO
knowing what the nitrate level is and how much of a water change brings it back to the target range is more important than an arbitrary percentage. If I did 50% WCs that would be great but 25-35% seems to work in this tank (probably 50% stocked, med-heavily planted).
Come to think of it, the water change pail I've been using strongly resembles the 5 gallon pails they sell at hardware stores.