Somehow, VPX magically figures this out. My project is set up as NTSC, 29.97fps. The 8mm film is scanned at 20fps using the Wolverine Movie Maker Pro, at full HD, or rather 1440x1080. A standard 50 ft film takes about 4 to 4m30s at 16fps.
Before doing this, I did some research about how to get the clip to run in VPX at the correct speed. In one video that I watched, the guy imported the clip into Windows Movie Maker set up for 30fps (or maybe 29.97) and then made a speed adjustment of 16/30 = 0.5333. He never mentioned the fact that the clip was scanned at 20fps by his Wolverine, or at least I think it was, because that is what the Wolverine does by default.
When I import a scanned clip into VPX, the clip is about 3m35s long. If I use 0.5333 as the speed adjustment factor, then the length would be way beyond what it is supposed to be and it would be too slow. But, looking at the Speed dialogue with no adjustment, purely with the clip as imported, the frame rate shows as 20fps, exactly what it was recorded at. The adjustment to 16fps is 0.80. Wow! So simple. I was expecting something complicated.
Of course, if there is too much flickering, which was a problem with the 16fps speed of 8mm film. one can always increase it slightly. I doubt that anyone would notice.
So, if anyone is concerned about speed adjustment for 8mm and Super 8 file and the Wolverine scan at 20fps, don't be. Magix makes it simple.
Now for the editing and cleaning up part. Neat video is a must.