ffmpeg to capture screenshot from a video file in a fine time unit

Question

I use ffmpeg to capture screenshot from video. Here is the command code:

ffmpeg -i /my_video_file_dir/video.flv -y -f image2 -ss 8 -sameq -t 0.001 
-s 320*240 /image_dir/screenshot.jpg

And I want to capture the screenshot at a fine time unit 8.344 for example

But it does not output screenshot image by the above command

In my test, -ss 1,1.5,2,2.5 ... works fine and others not such as 1.1,1.11

Does andbody know why it happens and how can I capture screenshot at a x.xxx time