Hi Clark,
You can use the Environment.CurrentDirectory property to get the runtime location of your process (Main.xaml). Then, you can concatenate the relative path with the current directory to save the file.
For your activities you can use also relative paths. For example, if you want to read an excel file stored in the same folder as the main.xaml, you can just specify the excel file name (the relative path) in the Read Range activity. But, if you want to download a file in IE, you need to use the absolute path in the Save As dialog (as the relative refers to IE application and not to UiPath process).
Best regards
Silviu