Updating excel from the web
For this web method call to work, put in a "m:" before the two cell Value parameters.
That is all you need for the Set Cell web method to work.
In this workflow/uda, I'm using one called Farm Admin. But I don't think you would be able to do this with the out of the box actions. It might require custom action or web service that uses the Excel API to open the document at let it recalculate and save it.
When selecting that web method in the Call Web Service action, behind the scenes, a SOAP packet is being built.