Opacity at camera- how to apply to per character 3d?

I have a text layer where I am animating the letters with per-character 3d. I want to wipe them off sequentially using Y-rotation. I set Y-rotation to -180 and adjust the offset. Of course this just rotates the characters to their back side. So I add opacity to the animator. But I still see the back side of the text before it goes to zero opacity.
I would like to apply this opacity expression somewhere so when each character turns away from the camera it disappears.
if (toCompVec([0, 0, 1])[2] > 0 ) value else 0
where would I apply this?

Thanks

At 90° characters in the center of the comp are exactly perpendicular to the view but others at the edges are not and you would still see them at an angle. doing a full 180° rotation with an expression on opacity would insure that the back face of the character is not visible.

Thank You