Header tank question?

the header tank is the smallest ..but in most cases very little water is drawn off the header tank to fee..d a system ..that's not the way to go ...so drain some water out of a rad ..holding ball valve up in header tank ..you do not want to add water yet ..before this you add the fernox to header tank ..give it a mix ..then do the draining bit ...all water in tank should then get into system ..then refill tank by releasing ball valve ...but pinholes in rads is not good ..either over pumping and getting oxygen into system or electrolysis