It's not Construct 2, it's just simple math, I'll explain
Let's say I have this diagram
|--+--|
In it, we can see that the + is in the middle, thus yielding to 50% opacity or 0.5 * MaxOpacity (100 in our case)
But how do we get that 0.5, what I did is
(Distance from start to +) / (Distance from start to end)
Now let's see out diagram:
|--+
~~^ (Distance from start to +) - 3 characters
|--+--|
-~~~~^ (Distance from start to end) - 6 characters
Now: 3 / 6 = > 0.5 <
Here ya go ... That's how programmers think