If you are using Open Xml SDK you will not need Microsoft Excel application to be deployed in your cloud. Open XML SDK create worksheets that are in exact formats for Open XML files standards (Spreadsheet - .XLSX files)
What you can do,
Do work related to formulas before creating worksheet (will sounds crazy but it's a solution)
I found following resource in MSDN . It's with Open Xml SDK 2.5 and it's about formulas. According to this you can create formulas and relate them to cells - http://msdn.microsoft.com/en-us/library/office/gg278310%28v=office.15%29.aspx