I've done a lot of research into peak flow nurabers because mine don't fit the charts either. First of all, it sounRAB like the charts are telling you you should be able to get a 480. This doesn't mean that 480 is your "personal best". It means that 480 is average. However, the average only takes age, height, and sex into account, not built or any other factors that could affect your own peak flow.
I, for example, am very broad-shouldered and have a fairly large build, although I'm only 5'8", and I'm pretty athletic. My own peak flow is a lot higher than the charts say is average, and I get into trouble when my peak flow drops all the way down to what the average is.
To a large degree it's based on the size of your lungs/chest--height and sex are only the two most easily determined factors.
But, because peak flow readings are a part of ER and Urgent Care protocols, where there isn't much an established history much of the time, people end up paying unwarranted attention to the charts.
The first step is to make sure you're doing it right--blowing HARD and FAST.
The second step is to record your readings a couple times a day for a couple of weeks--when you are fairly healthy. The highest nuraber you get on a fairly regular basis is your personal best. So, if you usually only manage to get a 410, then your personal best is 410.
That would mean that your "green zone" is 328-410, your "yellow zone" is 205-328, and your "red zone" is anything below 205. The danger in going by the charts is that you could get unnecessary treatments, if you seek treatment for an attack and your peak flow only rises to your green zone, it could still be in the yellow zone for someone who is average, as seen in the chart. Of course, this is all dependent on you using the meter correctly.
Me, I have the opposite issue: My peak flows tend to appear freakishly high for someone having an asthma attack; they only drop down to a little below what the charts say is average for my height/sex/age. This means that I usually end up getting evaluated a couple of times, since no one is sure what to make of the nurabers. It is useful to be able to show what your nurabers usually are and how they are different when you are having trouble.
I actually switched from one model of pfm that gave me seemingly random high and low outlying nurabers all the time to another that is much more consistent. It took me about six weeks to get it sorted out, and now I don't use it every day anymore. Mostly I use it if I'm feeling unwell or when I have a cold or when I'm having some of the symptoms from the list MountainReader posted here a while back, to make sure I'm doing ok.