I would consider 450 relatively 'okay'
But I assume you have been using one of these for the doctors to find out what is wrong? So you're best bet is to ask them!
I have breathing problems stemming back to when I was a baby, now I am being investigating by the doctors for possibly having a mild or adult cystic fibrosis. On average I blow 200 on a peak flow, sometimes as low as 100. And I'm 20 years old.
My 75 year old grandmother has COPD and asthma and still blows higher than 100!