You're welcome. FYI, since it appears that this is being executed from the [Asset Details] form, you can bypass the Forms commection and just reference the controls directly:
=DMax("PMEventUsage","PMEvents","AssetID = " & Me.AssetID)+DLookUp("PMInterval","AssetCategories","AssetCat ID = " & Me.AssetCatID)-DMax("Usage","AssetUsage","AssetID = " & Me.AssetID)