If your equation is $y=a \cosh (\frac xa) +c$, the minimum is at $x=0$ and the poles are at $x=\pm 35$. The sag is then $y(35)-y(0)$-the vertical distance between the support and the low point in the middle.
If your equation is $y=a \cosh (\frac xa) +c$, the minimum is at $x=0$ and the poles are at $x=\pm 35$. The sag is then $y(35)-y(0)$-the vertical distance between the support and the low point in the middle.