Steve,
So much depends on (a) the observer and (b) the type of scope used that I would regard this project as a waste of time. Sissy has seems blissfully ignorant of these factors. Andrew James did this topic to death decades ago - there a table by Robert Atkins which lists this.
http://www.southastrodel.com/Page029e.htm
There is also a formula for unequal doubles, she's not doing anything original.