Prepare your text extraction profile
For the file format of your choice, set up the QA options for length control, to manage the size of your segments. This will allow you to setup a QA alert to inform the user he will need to adapt the text no to reach the limit you have defined for each segment.
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You can even combine this rule with the Global Setting in the Translation Editor to prevent users from keep typing text, so that it is impossible to enter any additional character in the segment.
Work in a suited environment
It would be almost impossible to validate a subtitle without having it previously played in their final context. Subtitles are ment to be and additional layer in a video and Wordbee makes it possible to review them directly in the video module for subtitle preview.