You can't create forms with calculations in the Formscentral authoring application. Additionally there is no support for calculating fields in the HTML filling experience within Formscentral. You can however create forms with calculations in Adobe Acrobat XI and then host them on Formscentral.
This definitely works. I created a form in forms central, then saved it as a pdf. Then opened the pdf in acrobat pro XI and resaved the form using the menu item "save a copy". Then open the copy and you can edit it and make fields into calculated fields that calculate on entered fields in the form.