How are your lungs?

What is the average peak flow reading for a 25-30 yr old male?

I went to a walk-in clinic the other day after feeling a lot of tightness in my chest, but the nurse there said my peak flow reading of 500-520 was completely normal. Looking at that graph, it seems quite low....

When I was actually fit I was pushing 750. 500 sounds about right for an average person. Before I had my asthma treatment and breathing exercises I'd struggle to get out 350