I'm not sure why you have SUM's in your current expression, as the sample data you've provided is a single line. Therefore you're summing a single number, which always results in the same number. You can remove these.

As for your average, are you doing this average at a group at the bottom in your Dataset Group? If so, simply encompass the equation you have with AVG() in the grouping and it'll provide the result you want (in this case, 65.498%). So:

=AVG(Texbox1/(FaceTime-Texbox2))