How can the $\frac{80}{20}$ rule be applied in email marketing?

Posts should be $80\%$ "fun" and $20\%$ "buy my product"

Posts should be $80\%$ "buy my product" and $20\%$ "fun"

Posts should be $80\%$ "free" and $20\%$ "paid"

Posts should be $80\%$ "useful" and $20\%$ "fun"