You can create a macro in your Personal.xlsb. If you don't know whether that is active, follow these steps:
Start the Macro Recorder and in the "Record Macro" dialog select "Personal Macro Workbook" in the drop-down for "Store macro in". Set the name as ZoomLevel80. Hit OK and set the Zoom level to 80%. Stop the macro recorder.
Now hit Alt-F11 to open the Visual Basic Editor. On the left, find the VBA Project (Personal.xlsb). In the "Modules" section you will find a module with the macro you just recorded, but we can leave that alone.
Select the "This Workbook" module and in the code window put this code:
Private Sub Workbook_Open()
ActiveWindow.Zoom = 80
Click the Save icon. Now close Excel. The personal macro workbook is loaded every time Excel starts. The macro code above will run every time this macro workbook is opened.
The Zoom level is stored with each workbook, though, so when you open another workbook that was not set to 80% zoom, you can use the recorded macro. Assign a keyboard shortcut to the macro: Click the "Macros" icon on the Developer ribbon. Select the macro ZoomLevel80, click the Options button and enter a keyboard shortcut, for example Ctrl-8.
The screenshots have been taken with 2010, but the same principle applies in 2013.