Your peak flow meter readings depend on your height and weight and age.
Well if my peak flow got to 400, I'd be thrilled (that would be good for me at my age). Yes, it does depend on your age, etc. what the normal peak flow would be. Red flag is bad, time to change the asthma medicine or go to the doctor. Yellow is caution, okay. Green is you're doing real good on the peak flow. Mine was bad in the red most the time until I got some new medication from the lung doctor at the hospital.
I had two pulmonary function tests already showing that I had mild asthma, yet had been told it was chronic asthma.