Peak flow

Hi. I failed my first medical due to my peak flow reading. Is there a min pass reading??? The doctor has told me I need to get a reading of 712. I'm 16 years old and 179cm. Is there a scale for age and height??
Can't find any answers online. I've never had any asthma problems and my own gp seems to think that reading of 712 is high.
Any help appreciated.
Thanks. I've read through a lot of threads but there doesn't seem to be an answer as to what a pass reading is...there must be written criteria as to what a pass reading is and what a fail is.. nhs charts show a reading of 532 for my age and height but do the navy require a higher reading than what is average??
I just wondered if one doctor would pass someone on a reading of say 650 as an example but I could fail as the doctor I saw wants 712??