It took me minute to wrap my head around this, i’ve never used the key() before. After testing I was able to get this to work well for me.
p = thisLayer.transform.position[0];
k1 = key(1).value[0];
k2 = key(2).value[0];
v = k2 + effect("Slider Control")("Slider");
[linear(p,k1,k2,k1,v),540]