Hokum, probably a more practical formula is to base it on your height. A loft should really never be taller than you can reach comfortably.
One of the most impressive lofts I ever saw was a 10 X 10 building with a flypen that consisted of 1 X 1 hex wrapped around four large pine trees. The flypen was at least 25 to 30 feet high and it was STUFFED full of Fantails. It looked mighty impressive and the birds had tons of room, but it was not practical. At the time, the owner was selling out and had quite a line of people wanting these Fantails. It was quite a spectical watching this guy try to catch them, especially when someone said "I want THAT bird right there"!
One of the best lofts I ever had was very well planned out as far as venilation, lighting, hardware, segregation of sexes and a feeding aisleway - but it had one fatal flaw: It was originally a fruit stand and the ceilings sloped from about 10 feet in the front to about 8 feet in the back. This wound up being totally impractical, because if I wanted to catch a certain bird, half the time I couldn't reach it. If they flew up and hung on the wire ventilation window at the front, I'd have to jump to try to grab them.
I know lots of people who build their lofts with the idea in mind of being as tall as a sheet of plywood (8 feet) with the idea that less cuts and less wasted materials (hence maybe less expense and less work) is involved, but unless you are really tall, it is probably not the best way.
Needless to say, it is far better to consider your own height when planning a loft opposed to utilizing a general formula. Tall lofts for tall guys, short lofts for the short girls and everything else in between.
One of the best planned lofts I ever saw belonged to a friend of mine and it was the culmination of his 40 odd years of experience at the time. The roofline was less than 6 feet high, but he was only about five foot five. I used to just about tear my head off in there, but for him, it was absolutely perfect. He based the height of the interior off about the height of his elbow if he reached up straight over his head, so it was an odd height. The sections were also designed in such a way that if he stood in the middle of one, he could easily catch a bird anywhere in the loft.
Another downfall of overly tall lofts is also that the doors tend to also be too high, which can result in birds flying over your head and out the door when you walk in. You can remedy this by hanging a burlap bag or part of a tarp over the inside of the door that hangs down to about chest level or so (don't use a piece of scrap plywood, because eventually, you'll forget to duck and knock yourself out!) but this is usually a problem in lofts that were built too tall.