This screencast gives an overview of how to use Twixtor within Final Cut Pro, in order to lengthen or shorten a sequence to fit into a specified gap using the frame number option in Twixtor.

Timestamps:

00:34 Frame Number option to shorten clip

02:07 Slow shot down

**You can find additional and/or newer tutorials for Twixtor in Final Cut Pro here: https://help.revisionfx.com/album/28/