[Diego Villarino]

Thank you Dan,

I was trying to avoid any extra complication in order to automate the project for any kind of user. So the plan was to automate the info linked to the comp name and having only two kf for the whole animation. Anyway your response gave me the perfect hint and I ended up with this expression that seems to do the job (at the end I used a slider to control the box width, that is why I used one dimension):

L = thisComp.layer(index-1);
t1 = L.effect(1)(1).key(1).time;
t2 = L.effect(1)(1).key(2).time;
w = L.sourceRectAtTime(t2).width;
m = effect(“Margin”)(1);
an = w + m;
linear(time,t1,t2,0,an);

Best,

Diego