Hello, I'm tying to export a table of images to a quicktime movie with a set duration (in this example, 12.67 seconds)

atable = Table[
   Show[
    ListLinePlot[{dataSet[[a ;; a + w]], filtered[[a ;; a + w]]}, 
     PlotRange -> {-3, 3}],
    Graphics[{Red, PointSize[Medium], 
      Point[dataSet[[Round[a + w/2]]]]}]
    ],
   {a, 1, Length[dataSet] - w - (Length[dataSet] - Length[filtered]), 
    1}];

I first tried setting the "Duration" to 12.67 seconds, and got a 1:05 video instead:

Export["~/Desktop/tableAnimation.mov", atable, "QuickTime",  "Duration" -> duration]

(Here duration is 12.6)

Then I realised Export aways set a default framerate of 15 FPS (with 979 frames, 15FPS -> 65 seconds)

So i tried t set the framerate to 979/12.675:

Export["~/Desktop/tableAnimation.mov", atable, "QuickTime",  "FrameRate" -> (Length[atable]/duration)]

But I always get an 11 second video (85.71 FPS instead of about 77FPS)

Note that when I set the framerate to 30 (just for a test) I actually did get a 30 FPS video

Why aren't I getting the framerate I am asking for ?

Thank you