Currently we only have a few options when choosing at which frames we want to interpolate: 23.976, 24, 25, 29.97, 30, 50, 59.94, 60, 90, and 120.
But in terms of quality, the best that can be done is to interpolate 2x, without setting a specific output FPS. This way the amount of interpolated and native FPS is exactly the same, so no artefacts occur. That is, if I import a video with a frame rate of 23.97602 I should be able to interpolate 2x only so that the output video is only exactly 47.95204fps, not one frame more and not one frame less. This way you get the cleanest interpolation possible, without having to interpolate already interpolated frames to get to 60fps or 50, and there wouldn’t be as many interpolation artefacts.
So please, add an option to interpolate 2x only.