Create A Column Group consisting of more than one column

I have a table with the following format ;
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The Total Amt column sums from Monthly Rental to Other Charges. My formula is thus; " Sum(thisRow.[Monthly Rental],thisRow.VAT,thisRow.[Monthly Levy],thisRow.Utilities,thisRow.Maintenance,thisRow.[Late Fee],thisRow.[Other Charges])"
Using the above table as an example, I am proposing adding the ability to add the 7 columns in the above formula to a “Column Group” on which I can perform calculations. e.g. If I name the column group “MyItems”, the formula to calculate the Total Amt becomes " Sum(thisRow.[MyItems] ". In future should I add another column to “MyItems” Column group, I don’t have to edit the formula as it will automatically calculate for everything added to the column group.