Some work items have estimate time, completed time and remaining time but out the box they do not have any defaults.
When editing the Work Item templates using Powertools you can set a default and you can also set a formula (although it seems to be a dropdown list).
How would you set the "remaining work" default value to be Estimated - Completed?
The field formula that is displayed in the field definition window is for other purposes. You can use it to set the forumula that is used by TFS Reporting Services.
It is not possible to set a calculation for remaining work in the workitemtype. You could accomplish this by writing a custom TFS event that is raised when you change the completed work.
I hope this answeres your question.
Regards, Jeroen Niesen