RETRO, I hadn't read that link before I posed my question as I got stuck trying to read the link in your previous post.
Your second link mentions a 300-page report with a link to it so I had a quick skim of it. It devotes a section to the effect the use of guns has on crime and the various ways it can be measured. It is very technical and I would be lying if I said I understood the maths behind it.
However in the summary it says, "In summary, the committee concludes that existing research studies and data include a wealth of descriptive information on homicide, suicide, and firearms, but, because of the limitations of existing data and methods, do not credibly demonstrate a causal relationship between the ownership of firearms and the causes or prevention of criminal violence or suicide."
What they're saying is they can't tell either way. The calculation given in your link, is one figure and it may be backed by sound reasoning but I have no idea.