Can I measure time by counting frames and trusting on the ‘240 FPS’ that my iPhone 7+ slow motion camera is capable of record?

I’m using a slow motion video recorded using an iPhone 7+ to track something but would like to avoid recording a chronometer to know the time the process is taking. I need to measure about 10 seconds with an uncertainty of at most 0.1 s… Is this possible by just counting 2400 frames of my homemade video?

ffmpeg pipe from Openframeworks OFXVideoRecord, frame= fps= q= size= even when done until i kill Xcode process 2nd time

I am using OFXVideoRecord and hacking it for offline video recording

execThread: starting command: bash –login -c ‘ffmpeg -y -i ../../../data/beat.wav -r 30 -s 1280×720 -f rawvideo -pix_fmt rgb24 -i “/Users/mikegao/Documents/Visual/shapes/bin/data/ofxvrpipe0” -r 30 -vcodec mpeg4 -b 3200k -acodec mp3 -ab 128k “/Users/mikegao/Documents/Visual/shapes/bin/data/result2019-04-30-03-52-00-078.mp4″‘ &

If I leave out the first -i beat.wav, it generates a regular video just fine and closes after its done. If I add the -i beat.wav, it will keep printing frame= 3687 fps= 38 q=26.9 size= 50046kB time=00:02:02.86 bitrate=3336.8kbits/s speed=1.26x
frame= 3687 fps= 38 q=26.9 size= 50046kB time=00:02:02.86 bitrate=3336.8kbits/s speed=1.25x
frame= 3687 fps= 37 q=26.9 size= 50046kB time=00:02:02.86 bitrate=3336.8kbits/s speed=1.25x
frame= 3687 fps= 37 q=26.9 size= 50046kB time=00:02:02.86 bitrate=3336.8kbits/s speed=1.24x
frame= 3687 fps= 37 q=26.9 size= 50046kB time=00:02:02.86 bitrate=3336.8kbits/s speed=1.24x
frame= 3687 fps= 37 q=26.9 size= 50046kB time=00:02:02.86 bitrate=3336.8kbits/s speed=1.23x
frame= 3687 fps= 37 q=26.9 size= 50046kB time=00:02:02.86 bitrate=3336.8kbits/s speed=1.22x
frame= 3687 fps= 37 q=26.9 size= 50046kB time=00:02:02.86 bitrate=3336.8kbits/s speed=1.22x
frame= 3687 fps= 36 q=26.9 size= 50046kB time=00:02:02.86 bitrate=3336.8kbits/s speed=1.21x
frame= 3687 fps= 36 q=26.9 size= 50046kB time=00:02:02.86 bitrate=3336.8kbits/s speed= 1.2x
frame= 3687 fps= 36 q=26.9 size= 50046kB time=00:02:02.86 bitrate=3336.8kbits/s speed= 1.2x
frame= 3687 fps= 36 q=26.9 size= 50046kB time=00:02:02.86 bitrate=3336.8kbits/s speed=1.19x
frame= 3687 fps= 36 q=26.9 size= 50046kB time=00:02:02.86 bitrate=3336.8kbits/s speed=1.19x
frame= 3687 fps= 35 q=26.9 size= 50046kB time=00:02:02.86 bitrate=3336.8kbits/s speed=1.18x
frame= 3687 fps= 35 q=26.9 Lsize= 50228kB time=00:02:02.88 bitrate=3348.5kbits/s speed=1.18x

until I press stop in Xcode a second time (to kill the other command on other thread probably), then the video is written perfect:

video:48220kB audio:1920kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.175411%