Scale Gameobject Dynamically to Match Oval Object

Maybe the title misleading, but I don't know what the exact sentence to state my question correctly.

Look at below image :


Basically, I want to move the orange rectangle, which is pointed by red arrow, to the direction pointed by blue arrow.

Also, while the rectangle is moving, I want to continuously scale down its width to match the grey oval width on the rectangle Y position (depicted by rectangle duplicates above rectangle that is pointed by red arrow). It'll also have 0 width on the topmost of the grey oval.

>> What's the best way to calculate that (mainly the oval width on certain height)?

Thanks a lot guys!

P.S. Think all of them as 2d objects only