Show Total at the bottom of Grid from a Display Method column?

Hii,

I want to show the total or sum of all lines in a grid, but the column is a display method.

I followed this article of Martin Dráb and successfuly showed total of CustInvoiceJour InvoiceAmountMST field in the grid bottom.
http://dev.goshoom.net/en/2015/11/summarized-values-in-ax-form/

But don't understand how to do it with a display method?

Here is the detail..

Form Data source

This is my form. Here Balance column is CustTrans remainAmountMST() display method. I want to show total of Balance column at the bottom of grid.

Thanks,