What angle should I set the VTA on my VPI turntable?

I can't believe after all these years, I am asking such a basic "analogue 101" question, but here it goes. I own a VPI turntable that has a "VTA on the fly" knob.  I thought the best VTA setting was for the arm to be 100% parallel to the record surface.  

However, based on some research, I am not so sure that is correct way to set the arm to achieve optimal VTA and correlatively, optimal SRA.  Not sure, ... but I think I have to raise the pivot side of the arm.

Any advice would be appreciated. 

Thanks.