Hello SVS123,
Thank you for your post and question.
This method of destination file size calculation is very inaccurate.
Sometimes video have several video/audio streams, but after editing it is possible to save only one video/audio track. In this case this method will not be suitable.
Also there is not always the direct dependence between the file size and duration, the outut file duration depends on the format of the container and using muxer.