ok, so it seems if you use pure ulnar and radial deviation in the picking motion, this “side angle” of pick attack will be determined by the angle your wrist lays over the strings. The more your wrist comes over the top of the strings the more the side angle attack will be further from the perpendicular. If you went to extreme nonsense and your wrist approach angle to the strings was 0 degrees ( pointing down the barrel from the bridge to the nut ), then deviation alone will make the side angle pick attack at the perpendicular. But otherwise, once your wrist approach angle is fixed, It seems the only way to alter the side angle attack is to combine the deviation with some amount of pronation or supination.
I can bring the side angle attack closer to the perpendicular with the following:
On my downstroke which is ulnar deviation, I add some pronation.
On my upstroke which is radial deviation, I add some supination.
This is the motion I have been doing attempting to do cross picking, which I also string track on. It is where I noticed the difference in the side angle attack versus my pure deviation picking… and then started detailing it…