Use the higher reading from the quick sharp blow.
Quick sharp blow, that is the right method.
You may have seen this already but WebMD has a section on peak flow meters and instructions on how to use them. Doesn't really answer your specific question though, but does explain that doing it the same way is what is important.
The best is to make sure you are blowing in the same manner each time, that way you know when your volume changes.
The number itself doesn't mean anything, it is the drop or change over time that the peak flow is helpful for.