As opposed to 600 finds reported by 60,000,000 members of the public (nigelswifts figures)
How many finds are unreported using these figures?
Your reasoning is sound, it's the math that I find iffy.. :-)
60,000,000 people aren't roaming the countryside with equipment designed to find buried metal. If you provide figures for members of the public actively looking for artifacts then I would be happy to do the maths for you.