Timing Rule of thumb

[Bohdan.L.Bodnar at att.com]

I never read or heard about this rule-of-thumb.  I do know that knock will
depend on:

1). coolant temperature
2). incoming air temperature
3). a/f ratio
4). EGR flow
5). fuel's octane rating
6). combustion chamber design (e.g., swirl-head design is less susceptible to
    knock than other designs)
7). valve overlap

As an aside, what temperature scale was used?

Have a nice weekend!

BB