Actually it is easy... so nothing to dislike, I just did not get it, math classes are ages ago
If anyone is interested:
Since the solution is sin(pi*0.5*180/pi), which us equal to sin(x*90), just multiply any value x by 90 placed into the sin function to get the rad value...
It is nice to use instead of easing (e.g. a fade or motion) with a plain lerp function, since you can predetermine the exact time duration of the fading.
lerp (MinValue, MaxValue, sin(TimeEllapsed*90*1/FadeDuration_in_Sec))