If you are trying to blend a sunset in the background with a portrait, you need to make a sunlight-balanced fill flash exposure. The Kodak book
Using Flash does a good job of illustrating this and gives some useful starting points for the flash value relative to the ambient light.
Amazon.com: Electronic Flash (Kodak Workshop Series): 9780879853723: Lefkowitz, Lester, Eastman Kodak Company: Books
A good starting point is to set the aperture and shutter speed on the camera for the ambient light. Then add about one stop less flash exposure to properly record the portrait subject without overpowering the light from the sky. This is referred to as a -1 stop fill flash. Done properly, the results can be quite beautiful.
Its useful to make fill flash exposures of -2/3, -1, -4/3, and -5/3 stops relative to the ambient exposure. This is most easily done with a TTL automatic flash. You need only set the film speed on the flash unit to the appropriate speeds. Of course, the shutter speed must be no faster than the flash synchronization for the camera’s shutter.
For example, suppose that you’re using ASA 100 film. Set the camera’s meter to ASA 100 and choose an aperture that will work with the flash’s aperture limitations. Let’s suppose this is f/5.6. If you want a -1 stop fill flash, set the ASA speed on the flash unit to ASA 200. That will give a correct ASA 200 exposure of the subject that will lighten the subject, but not overpower the ambient light portion of the exposure.
With ASA 100 film in the camera, you’d use the following ASA film speed settings on the flash for fill-flash exposures:
160 for a -2/3 stop fill
200 for a -1 stop fill
250 for a -4/3 stop fill
320 for a -5/3 stop fill
400 for a -2 stop fill
This can also be done with a sensor-automatic flash. It’s a bit fussier with a manual flash, but can be done. You might need to use a flash meter so that a manual flash can be positioned appropriately relative to the subject to harmonize as wanted with the ambient light.
The flash portion of the exposure will produce normal color balance with a 5500K daylight balanced color film. The warmer colors of the sunset will look approximately the same as you see it. The result can be charming.
For best results, do an informal set of test shots with your equipment so that you can sort out any problems prior to the actual portrait session. You can then choose the particular ambient-to-flash combination that works best.