Consider a function $$f$$ satisfying $$f(x*y) = f(x) + f(y)$$ where $$x, y$$ are positive integers, then what is the value of $$\dfrac{f\left(3\right)\ +\ f\left(12\right)}{f\left(12\right)\ -\ f\left(4\right)\ +\ f\left(2\right)}$$?