I saw a form like this in an old thermodynamics paper yesterday:
$\frac{1}{t_2-t_1} \times \frac{y_2-y_1}{(y_ \infty -y_1)(y_ \infty -y_2)}$
where $y_ \infty = \lim_{t \to \infty }y$
It had to do with estimating reaction rates, but there was no explanation.