Completion setting

My suggestion is similar to RodWard's.  You can add a slide at the end of the video that is the last frame of the video.  Viewers won't see a change as it is part of the video. Then set the completion setting in the SCORM quiz settings to 2 slides for completion.

Another option would be to have the video spread across several slides.  (Again the viewer won't see this)  Then set the slide completion to equal enough slides to get a 90% completion.   For instance spread the video over 10 slides and set the completion to 9 slides.

I would suggest you should have a slide BEFORE the video slide, not just having a single slide module.  It is never a good idea with Captivate to put heavy content (e.g. a video) on the very first slide of any project.  The initial slide doesn't need to be very long (e.g. one second), but that slide is essential for making sure the Captivate interface and all necessary system variables are set up BEFORE the heavy content starts.

One way to allow users to complete a module after completing 90% of the video would be to add an Exit button on top of the video at about 90% of it's timeline duration to give the user the opportunity to exit at any time after that.  Clicking the button could then either report a score to the LMS that gives them a Success on the module, or it could simply navigate them to another slide AFTER the video that allows the module to quickly reach the end of its timeline and end playback.  

Another option is to add that other slide AFTER the video and set the SCORM settings to require the learner to view at least two slides.  (Remember I suggested a slide BEFORE the video.)  However, the learner needs to be told in advance that they must view the module all the way to the end of the video otherwise they may bail out during the video thinking that by getting to the video slide they have done enough to achieve completion.

If using buttons to report to the quia, the user doesn't need to know the button has a score.  They just need to be convinced that clicking that button is the ONLY way they can complete the module.  So you need to provide instructions somewhere to dissuade users from just closing down the browser window without clicking the button at the end of the video or on the final slidle, because doing so would mean no score and no completion.  Adult learners aget really unhappy if you don't tell them advance warning about circumstances that might cause them to fail at something.