Philip Greenspun , Oct 02, 1996; 08:11 p.m.
The most complicated thing here is that the A2/EOS-5 cameras will automatically reduce the flash output by about 1.3 stops when the ambient light is "bright". You can disable this with a custom function. It is documented in at least the EOS-5 manual.
My main suggestion is to do everything manually unless your subject will be more or less smack in the middle of the frame. The Canon flash metering system sucks. Nikon got it right with their D system.
Anyway, if your subject is front and center then all you really need to do is set the exposure compensation on the back of the flash to -1 or -2 stops or whatever. Then the flash exposure will be calculated automatically.
The Kodak Professional Photo Guide has some useful tips on fill flash.