Given a linear function such as
$y = 1.62*x - 0.49$
If $x = .5$ then $y = .32$
If we then increase $x$ by $10$% ($x=.55$), then $y=.401$, which results in $y$ increasing by $\approx .25$%
If $x = .6$ then $y = .482$
If we then increase $x$ by $10$% ($x=.66$), then $y=.579$, which results in $y$ increasing by $\approx .20$%
I am looking to explain how an improvement in x results in an improvement in y. Clearly, the relationship is not linear. I do not remember enough calc to model this relationship, if that is possible. I am looking for a mathematical way to say "as you improve x, you will see an improvement in y; however, if x is already high then an improvement in y is less impactful. Therefore, increasing x has the affect of improving y, but the return on investment is greater if x is initially very low." or "if x is bad and you improve it you can expect this return in y, but is diminishes as the starting x is higher"
I will also allow that my original premise is faulty. Thanks!