I am not a nutritionist and unfortunately cannot help you determine your optimal iron intake per day. I'm afraid that the Fitbit won't be able to help you either.

I have Chance 5 and the upper values in the app are in mg and the lower values are in percent....
What percentage in the app is the value of iron or vitamin c that I am allowed to have daily, which I can follow?

Can you be more specific with your question? I don't see the connection with the Fitbit...