Upload & Sell: Off
Not sure i understand your question workerdrone but i'll give it a try anyway;
Lest say your flash is first mounted on your camera and you have set you flash to TTL and your camera mode in Aperture priority. Suppose you chose f4, then as a default setting the camera will choose 1/60th sec shutter speed and will give you and ok average overall exposure. If you don't like it you simply dial the flash output up or down a little to have it to your liking. So you have a starting aperture and speed setting for a given iso. What you don't know is the exact flash output it took to get the shot the way you wanted it. The way i see it there's 2 ways of going about the problem of reproducing this shot going manual; Either you deport your flash on tripods and you shoot a few shots at the same camera settings and play with you manual flash power until you come back to you starting results by eyeballing you view screen or you use a flash meter (if you have one) and measure you flash output while still in TTL mode. Once your set up changed in manual you simply dial your flash output until you get the same reading you had on your flash meter.
Other than that i don't know what to tell you, there is a possibility of using the FV lock (flash value lock) function on certain bodies such as the D3s where essentially by pressing the shutter halfway while pressing the fn button the flash value will be memorized, but that is only useful when you want to recompose in such a way that your subject is moved away from the center of the image but when you still want the same flash output as opposed to lighting the background with your flash burst which would overexpose you foreground subject.
Hope this helps a little, maybe other people have better ideas. Good luck