That sort of rule of thumb is going to be most accurate for average salaries, because large amounts of the non-salary costs are relatively fixed (e.g. health in

That sort of rule of thumb is going to be most accurate for average salaries, because large amounts of the non-salary costs are relatively fixed (e.g. health insurance costs the same amount whether you earn $50K or $200K annually). As salary goes up, so does its share of total compensation.