Hi Shaggy,
The best method to establish the temperature to reharden at is by using a magnet. The steel will become non magnetic once the bottom end of the critical temperature zone is reached. If you go above this by around 200 degrees you will obtain the full hardness without the guess work. To soften the steel is basically the same without quenching. If you have access to a heat treating furnace you will be able to ring the steel up to temperature in it then allow it to cool in the furnace.
It's already been menioned that some files are only case hardened and it is a good idea to cut off a small test piece to save the heart ache of putting in a lot of work for nothing. This has happened to me nn the past.
If it does turn out to be case hardened it can be used for damascus projects down the track.
I hope this helps.
Keith.