I have 5 gallon carboys. I use one-stage fermentation. Fill the carboy up to 4 or 5 inches from the top. I put the Blow-off tube on when I yeast.
I always lose a couple quarts of beer, into the bucket. Beer stays in the carboy, about 10 days, then I bottle. It's a simple one-stage process.
I always have mellow beer.
'Til Later...Happy Brewin',