There is no config option as this is a worksheet specific setting.
To force Mathcad Prime to use your own preferences by default for every new blank worksheet.
You can either edit the file the that Mathcad uses when you click the New worksheet icon
The file that Mathcad launchs when you choose New worksheet is call Default.mctx
Located in the default templates directory (64-bit version)
C:\Program Files\PTC\Mathcad Prime 3.1\Default Templates\
Or
Create the document with your preferred layout, page, grid, orientation etc and save the mctx. file
Then you can point Mathcad to launch this file when ever you create New worksheets
To do this
Go to Mathcad icon and click on Options. Choose the Options Tab and check the option to
"Use an alternate template when creating new Worksheets"
Using the option to Browse and select the new blank worksheet.