In the world of cam specs, the "advertised" duration was supposed to be according to some SAE guidelines, which did not count "the ramps" before actual valve lift started. So, although most used the SAE guidelines, they were not something the enthusiasts had real access to OR they might not know if they were accurate unless somebody actually measured them. For example, one brand's advertised duration of 292 degrees might actually be "shorter" than another brand's 280 advertised duration camshaft. All of the vehicle manufacturers were supposed to measure their cams according to the SAE criteria.
To combat that, the aftermarket began the "@.050" duration spec. The duration after the valve lifter has reached .050" lift from the cam's base circle diameter (where a dial indicator would be set to "0"). No advertising mumbo-jumbo, just real numbers that mean something.
As a Mopar Performance operative once mentioned (at a MP Seminar at Mopar Nats one year, on this subject), the .050" duration on OEM Chrysler cams were about 80% of their advertised duration. Which was a rough guide, it seems. Only way to actually know was to put a dial indicator on a valve lifter, with a degree wheel on the crankshaft, and see what the degrees came out to be, with the factory cam.
Any of these durations need to be positioned against the cubic inches of the engine the cam will be installed in. Which is another subject.
Enjoy!
CBODY67