Pull path from current file directory

I'm Using Ghowl to write to a .xlsx file. I'm looking for a way to create a write path using the directory that the .gh file would be located in. 

When I try to use a path like "datasheet.xlsx" (as opposed to an explicit directory C:/datasheet.xlsx - I don't know the terms for these two types of paths), it writes to the location of the rhino file, not the grasshopper file. 

If you wondering why I need this i'm creating a definition that is to be used as a template, which will be saved in various locations. It then it creates a data sheet that will be saved to the same location as the new .gh file.