Zoho CRM reports: total from a formula

Zoho CRM reports: total from a formula

Hi,
 
I'm trying to customise reports from Orders. It works well but when I want to calculate datas from a numeric field but when I want to calculate datas from a formula fields from Order, it doesn't work. Is that normal ? Could someone explain to me how it works ?
 
Thank you very much,
 
Eric Marois