I shoot with manual exposure a lot at weddings, and other times, and the settings you have proposed would not be what I would typically use, either indoors or out. Take the following examples....
Outdoors on a sunny day : Consider a standard "Sunny 16
" as a guideline exposure of f/16, 100 ISO, 1/100. Your settings of f/8 (2 stops brighter than f/16), 400 ISO (2 stops brighter than 100 ISO), 1/125 (1/3 stop dimmer than 1/100) would overexpose the scene by 3.7 stops even without the flash. In fact you would need really quite poor lighting outdoors for those settings to be viable. Assuming you want to use flash for fill and you do not want to use high speed sync then for sunny conditions you would need an exposure for the ambient of around 100 ISO, 1/250, f/10. Any brighter would blow the background, and possibly (parts of) the subject. Of course, if you use HSS then you can open up the aperture, perhaps from f/10 to f/4 (2.7 stops) to isolate the subject a little from the background, and then happily increase shutter speed from 1/250 to 1/1,600 (2.7 stops).
In a church or registry office : Since conditions can vary so much I can only go by example settings I have used, when flash has not been allowed. Examples are....
- 800 ISO, f/2.8, 1/80 (that's 4.7 stops brighter than your original settings);
- 3200 ISO, f/2.8, 1/60 (that's 7 stops brighter than your original settings);
- 800 ISO, f/3.5, 1/60 (that's 4.3 stops brighter than your original settings).
Even with flash, you would need to bounce it to light up the room or you would end up with a very dark background. The flash would have to work hard and may overheat or suffer longer recharge times or simply be quite annoying for guests and officials.
Evening/reception : Again lighting varies but based on my typical settings, intended to preserve good exposure of the ambient/background I'd use something like....
1600 ISO, f/2.8, 1/60 + bounced flash (that's 6 stops brighter than your original settings);
3200 ISO, f/3.5, 1/60 + bounced flash (that's 6.3 stops brighter than your original settings).
Once again, unless your flash is doing magic (magic is possible but the room must cooperate) at lighting up the room you will end up with a very dark background. If the room is sympathetic to bouncing you may be lucky. If not you may be in trouble.
By and large I'd say that wedding togs do not invest in f/2.8 zooms and even faster primes so that they can spend all day at f/8. f/8 is not the answer, IMHO. You need to balance ambient and flash exposures to good effect and that means working both your manual settings and FEC as the conditions and environment change.
