You need to include the internal links in the equation - i.e., there is not just 1 backlink but 7. Bear in mind that the exact current value of d is not known and that there may (or may not) be a dampening factor for internal links, but that may at least get you closer to the actual result.
I appreciate you clarifying this for me. I've seen the same thing with other sites only having one external link and that is why I was confused with the equation.