KSM,
What color chart are you speaking of?
If it is indeed an old file, it is probably 1095, which is spring steel, but I cannot say. For a rockwell of 58 or so I would draw back at 400 degrees. Then TEST it. If it is too hard, put it back in the oven at 425 degrees for the perscribed two hours. Test again. If it's too hard bump it up 25 degrees again and so on.
Question: When I refer to testing, do we understand what that means?
It means to roll a cutting edge onto the knife. Get it sharp. Then cut some rope, chop a board, do the brass rod test, etc and decide if it is peforming to your needs. All of this is done before the handle is installed.
A bladesmith should be trying to get in a position where he can take a mystery steel (high carbon spring steel, file, etc)and heat treat it properly by working up the chart, if there is one, till he makes a blade that is servicable. I personally like to be able to do it, but dont hav ethe time to waste on mystery steels. Even so, I test every blade, every blade. This is to allow for varying conditions that might cause the heat treat to not "take". The goal is to remove variables and not add them with "unknowns". I tell you this so you guys will know why I and others here sometimes dont speak up. It's not because we're uppity. Speaking for myself, it's because I just dont do things that way or I cant recommend doing it that way or in the context of the question asked. I cant afford to. It does take some of the fun out of it, but the results are are hard to dispute.
For the most part we here on TG like to do this for a hobby in the spirit of tradition which is great and refreshing. That's why I'm here as well. Make it as fun as you can.