But I recommend doing it differently because the values aren't going to be consistent, for example, when you switch from positive to negative numbers. Instead, I recommend using a multiplier for the index and add (subtract because positive numbers are further back in z space) that. Something like this:
Now in this case, the second layer will always be 100 behind the leader layer. If you apply the expression to the third layer in the timeline, it will always be 300 behind the leader layer. If you want to make it further, just change the number in the multiplier to be higher. Also, make sure to change the name of the 'leaderlayer' to whatever that layer is actually called.