Don't double post.
After choosing action Unit - Make Unit Face Angle press the underlined field "Default building facing" and find 'Arithmetic' and the top of filter. Now, click on first parameter and find 'Unit - Facing Angle' - it will be set to 'Facing of (Triggering unit)' by default. Replace (Triggering unit) with your unit (the one which you want set facing angle to) and press okey. Then select the second parameter and fill it with "20.00". Press okey on everything to leave the trigger window panel. Done, enjoy.