In After Effects how can I generate the value of an opposite x position and then send that value to an effect on another layer?
I think what you're asking to do is simple using Expressions.
If you alt-click on the stop watch of any keyframe-able control you can write an expression to connect that control to another control.
So if you want to control the end of ramp's x position using the start of ramp's x position in your example, it would be:
x = 4480-(effect(&quot;Gradient Ramp&quot;)(&quot;Start of Ramp&quot;)[0]);
y = 1080;
[x,y];


The [0] indicates an array - it means &quot;take the x position of the &quot;start of ramp&quot;, which has two values, x and y, which can be called as [0] or 2 in the array.
To duplicate this in another layer, duplicate your first layer using cmd+D, then add an Expression to the start of ramp in the duplicated layer:

You can use the pickwhip to select the control you want to enter into the expression