expression, lock only scale the Y axis.

Scale is an array [x, y, z] for 3D layers. x [0], y is [1], and z is [2]. If the layers are 2D then all you need to do is use the s[0] value for x and the existing value for y.

Here's how to do that:

L = thisLayer;

s = transform.scale.value;

while (L.hasParent){

L = L.parent;

for (i = 0; i < s.length; i++) s[1,i] *= 50/L.transform.scale.value[1,i]

}

[s[0], value[1]]

Now any value you enter or even animate for y will not be affected by the scale of the parent.