When is a Peak Flow number too low?

I'm 49 yrs old, 6'2" non-smoker with Chronic bronchitis and sever asthma. 350 on the peak flow meter is a normal day for me. 150 - 200 is not a good day. 400 - 500 is a good day for me. The thing that has saved my life is the Xolair shots I get. The shot cost $8,000 a month but well worth it.