RInterpTo: How to calculate Interpolation speed between two Rotators

Hey guys,

I am trying to calculate the interpolation speed between two rotators. I have the initial rotation, target rotation and the time it should take to complete the interpolation. I am not sure if the calculus is correct. My current approach is the one in the following image, but it never takes the desired time.

I am trying to use this speed in the FInterpTo funtion in C++.

Thanks in advance