That's "Read the Flash Manual."
...that is, unless what you want to know is nowhere in the flash manual. Or any of the otherwise-good web references.
The question is: How do you use flash with the EOS when using manual, stopped-down lenses?
No simple E-TTL for Cantax, alas. Trying E-TTL with the Zeiss instantly resulted in a pile of underexposed shots. Hmm.
Well no problem -- just dig out the little Metz 28C-2 that I've used with my film Contax for years. It's a great strobe. Just set to "A" and set the lens to f/8 and.... hey! Nada. For reasons I can't yet fathom the 300D body simply refuses to fire the Metz. How am I going to use those Zeiss lenses with flash? Am I stuck with using manual flash exposure on the 550EX?
Fortunately, I noticed that the exposure at /8 was wrong but looked a lot like another wrong exposure at /1.4. A pattern emerged: On 550EX, E-TTL is off by three stops using manual lens adapters. The exposure will be approximately right if F.E.C. is set to +3 stops.
Of course +3 stops is the upper limit for the 550EX so if you want brighter flash than that, you'll have to go manual. But it's a start.
The built-in flash has no F.E.C. or manual mode, so am I stuck using the great-but-heavy 550EX? Nah. There's no F.E.C., but you can fake it aim at something three stops darker than "normal" exposure (that's Zone II for you old-school B&W types), hit the * button to lock the flash exposure, recompose during the next 15 seconds or so and shoot.
Or for manual exposure, use a variation on the same technique. The built-in flash for the 300D has, by my estimation, a metric Guide Number of 13 (39 in feet). To get the flash to deliver the full charge, put your hand over the lamp and hit * the flash will lock on full-power. Now focus, set your f/stop appropriately to the GN (distance 2 meters, 13÷2 at ISO 100 is ~/5.6 or so or /11 at ISO 400), and shoot.
BTW, the actual "normal" GN of the 550EX is not 55, as would be implied by the name. It's only 55 in telephoto mode. For normal-lens usage, the GN is really 42.
And while we're on the subject of the 550EX zoom head, why did Canon get the auto-zoom wrong on cameras with smaller sensors, like the 300D and 10D? When I put the 50mm Canon lens on the camera, the 550EX should really zoom to a format-appropriate 85mm, not 50mm. Even with the EF-S lens, the strobe gets a 35mm-appropriate dispersion. Whoops, guess I'll have to set it manually (or burn through some extra batteries).