How to show a calculated row in financial statement AX 2012?

Hello, how could we show net sales in P&L supposing that I have

Net Sales = Sales - Discounts

Sales include a range of accounts say 3000 to 3100

Discounts include say 3200…3280

I have groups that show the total for Sales & Discounts.

When I try to create a calculated field where I added these two group in the Arguments available and try to subtract row #1 - #2 the system throws an error. However, I notice that when I add dimension in to the arguments, it works!

appreciate if someone can help. many thanks

Do I need to create total account type say Sales Total, Discount Total from the Chart of Account? Appreciate your thoughts