The number of trajectories to go to an element of the Rk+dk,o+do subclass coming from the Rk,o subclass is :
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And the total number of trajectories of dk steps is 2dk
Each element of that subclass is such that :

So, an number of the Rk,o subclass has a probility :
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to have an expansion :
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If we consider that that event of probability p occurs for x around 1/p :

We can build a model for maximum expansion by :

Let us plot this model and real expansion records at dk, number of steps to reach the Maximum Height

Figure 1 : Maximum Expansion Model