Rules of thumb are not random values; they are simplification of more complex rules by focusing on the factors that make up 90% of the underlying equation.
For example, rule of thumb. Sitting president loses midterms.
Complexification: well let’s look at president approval rating, the economic growth rate, the unemployment rate and how congress is polling…
Sure the latter is more precise but the former just lays out the 100 years of experience in one simple metric.