My opinion would be YELLOW for optimum reading as you want the Flex Parallel to your antenna, HOWEVER you can also do that with the RED at the top / bottom of phone antenna but just not quite as much wiggle room for accurate antenna placement.
THAT BEING SAID
My Apex Flex is at #P0 - L
This is in effect the same orientation as your RED, I have zero issues reading it.
The physical placement for the Flex, IN MY OPINION would be better in the RED orientation.
I have Flexs in my forearms, and chose the orientation because I felt it would cause less stress on the flex, and theyt are easy to read.