Here's the loooooooooooooooong answer:
In reality, no matter what container you boil in, the percentage boiloff rate is a farce. What is relatively constant is the boiloff rate in terms of gallons per hour. For most homebrewers on "normal" homebrew systems, this rate is anywhere from 1 to 2 gallons per hour. The variables are surface area of the boiling wort, and boil vigor, i.e., is it just a simmer, or is the wort practically leaping out of the kettle, or somewhere in between. Personally I shoot for leaping. And, the wider the kettle, the faster the boiloff rate. Then in order to get your gallons per hour rate to work in your brewing software, usually you will need to convert that to a percentage, even though the percentage is not actually constant if you boil shorter or longer than an hour, or especially if you change your batch size. For instance, if you normally make 10 gallon batches and lose 10% per hour, you will lose somewhere closer to 20% per hour if you switch to a 5-gallon batch size on the same equipment. Fairly straight-forward mathematics. But if you always boil the same sized batch for the same amount of time using the same boil vigor, then this all becomes moot as you should have a constant percentage boiloff rate.
"This is grain, which any fool can eat, but for which the Lord intended a more divine means of consumption. Let us give praise to our Maker, and glory to His bounty, by learning about... BEER!" - Friar Tuck (Robin Hood - Prince of Thieves)