100-200 ppb of residual is ...

100-200 ppb of residual is enough. There are 2 factors:

• You have to be sure that there is residual in water
• You have to avoid overdosing (to save money)

And as Terry said iron content is a good indicator of correct treatment. So if you have for example 50 ppb of oxygen scavenger and 0 ppb of iron it is enough for your system and your scavenger.