Adding % of Total Column in Matrix When Using a Field That Is Grouped by Columns

Hello all,

I currently have a matrix visualization that I am creating in PowerBI to replicate an excel report. Based on what you can see below, the matrix uses a colum with date group labels (a column already supplied in the excel raw data reference table) called "Age First Close Group". What I am trying to do is add an additional column that provides a % of the total column that are represented in the 30 Days column. In Excel I would just use a simple formula to divide the value in the 30 Days column by the Total Column for each row and express as a %. I am at a loss as to how I can replicate this in PowerBI. 

Thanks in advance!