A rule of thumb is just a rule of thumb, I don't think anybody would say that a particular rule of thumb is exactly how you would value a website. Maybe we can generate a hierarchy website valuation by the type of site. I would guess (number of users/visitors being equal) that a website that simply serves as a manufacturer's billboard would be worth less than a blog generating ad revenue which would be worth less than a webapp generating revenue through ads which would be worth less than a webapp generating subscription income.