I need a formula to find the end angle of an elliptic curve with given start angle and length.

i.e. Consider an elliptic arc with major axis 100 and minor axis 50, if the start angle of the curve is 0 and the lenght is 70,5 so is there a way to calculate the end angle.

I`ve read an article about Jacobi`s studies on inverses of elliptic integrals. Jacobi Amplitude is the inverse of elliptic integral of the first kind, but I couldn`t find anything about the inverse of elliptic integral of 2nd kind, that could be a solution for this problem.

Would you please share your ideas with me to find a formula for this.
Thanks in advance.