How can I make a curve match another curve in Illustrator?

Question

I'm trying to make a very minimalist graphic of a cylinder shape in Illustrator. I've already made the basic shape with a rectangle and two ellipses, and now I'm trying to put a simple highlight on it. I've snapped the two anchor points on the end of the highlight path to line up on the edge of the ellipse. I want to then make a curve on that path that exactly matches the curve of the ellipse. How can I do this? I've included a few screenshots below of my artboard.

entire cylinder:

zoomed in:

further zoomed to show curves do not line up:

(Note: I realize that this question may have already been answered, whether here or on another site, but I was having difficulty wording my search in Google, so I decided to ask here. If so, please just link me to that answer.)

Answer 1

You can use the Shape Builder tool for things like this.

Draw a rectangle to overlap the surface of the cylinder, then using the Shape Builder tool, click and drag over the left hand side pieces to unite them. And hold down Alt+click to delete the piece on the right.

Example

Answer 2

I just figured it out! If you had the same question as me, the solution is actually quite simple. You just have to duplicate the cylinder, place it above the highlight, and then use the Intersect option in the Pathfinder panel. Below are some screenshots showing my process.

starting state:

cylinder duplicated and placed above highlight:

hovering over Intersect option in Pathfinder window with both paths selected:

end result (note that I had to fix the highlight's fill color):

[Edit: I just realized that placing the duplicated cylinder above the highlight is unnecessary. Plus, leaving it below also eliminates the need to fix the highlight's fill color after the shapes are intersected.]