How the parent node progress percentage is calculated?

Dear Gents,

Kindly your support to know the way of calculation of the parent node progress for the following highlighted percentage.

The normal behaviour for doing that is to get the Average of the child nodes to represent the progress percentage at the parent node.

So, Could you please demonstrate the default way of calculating the node percentage, and how can i get the Average value for its child nodes, as i have 3 level of nodes as the following:

Portfolios -> Projects -> Tasks

Thanks,

Mahmoud