|"His answer was that AMD sets their FSB default at 100"|
AMD has nothing to do with it. And 100 is NOT the FSB, it's the CPU frequency. The motherboard/BIOS is what dictated the CPU frequency to be 100MHz. The CPU was designed to run at 166MHz frequency but the BIOS defaulted to 100MHz. That's not uncommon. You should always confirm that the CPU clock settings are correct in the BIOS. BIOS defaults are never the best performance settings - they're not supposed to be. All they are is a starting point. They're also a fall-back point if you get in trouble.
"But what I'd like to know is where did he get that formula"
Formula? It's not a formula, it's the clock settings for that particular CPU. The Athlon XP 2800+ is designed to run at 2.083GHz which is 12.5 x 166MHz. 2800+ is the model number & has nothing to do with the core speed or clock settings.