Peak Flow Range

Does anyone know the formula for the optimum Peak Flow Range? I've heard some say BMI plays a part, I DO know you have to take in account age, height, and weight...in kilo's? Any help would be greatly appreciated! Someone told me to have a pt. take the best of three on the peak flow daily for two weeks and the average is their best. I understand that, BUT if a patient's asthma isn't under control, and they are constantly in the 240"s this may be their best. I just want to know how to figure what number they should shoot for!

Thanks!