I've got a solution for you.
I entered the following equation:
multiplier * sqrt(acos(1 ul - 2 ul * x / length) / 1 rad - sin(2 rad * ( acos(1 ul - 2 ul * x / length) / 1 rad )) / 2 ul)
Change the first two user parameters (length and height), and the equation curve will update accordingly.
The attached IPT file can be opened in Inventor 2013 or later.
Hope this helps,
sorry for this late response, but thank you for your reply! Your solution works perfect for me, though I don't know why Inventor has to be so difficult in this manner, but who knows.
Thank you again for your help, I will mark this as a solution.