keyframe expression respecting animation curve of original keyframe value

[Adam Greenberg]

Hi, I am having a problem templating a project for production. I will simplify the problem.

example

keyframe at 1st postition is (960,540)
second key frame is (937,540)

this second keyframe will change for every project, (922,540), etc.

neither keyframes are time dependant, they remain fixed at their position in time, so I have created a slider and added the expression below.

But my original animation curve is not easein, it is easein at first, but then the curve adjusted in the graph editor to exagerate the ease in. And Because there are other elements animated with this curve in the project, the slight difference is noticeable. If I change all animations to ease in, then the expression works as expected but this solution is not sufficient. The Client insists on the original animation.

Is there a solution with expressions? For now I have put instructions so that the user will absolutely have to be on the second keyframe before adjusting the position, and that way it is still a template, but I’d like to take it a step further if possible.

Thanks kindly for any help.

if (numKeys > 1){
s = thisComp.layer("Null 1").effect("Slider Control")("Slider");
t1 = key(1).time;
v1 = key(1).value;
t2 = key(2).time;
v2 = key(2).value;
ease(time,t1,t2,v1,[s,v2[1]]);
}else
value