Here is a link to a good description of four different approaches to hose routing...the "classic" alternate reg donate configuration and three primary reg donate configurations, each with routing starting under the right arm, differentiated by the primary reg's hose length: 40 inches without neck loop, 5 feet with neck loop, and 7 feet with neck loop.
Keeping Your Hose in Line - International Training - SDI | TDI | ERDI | PFI
I use the 40 inch primary reg donate configuration described in the link but with a 44 inch rubber primary reg hose (Piranha Dive Mfg) attached to the primary reg with an Omniswivel.
I would prefer to have the primary hose on a 5th port on the bottom of my Apeks FSR 1st stage but my FSR is too old to be compatible with the 5th port upgrade. Regardless, it works fine with the necklaced alternate reg on a 22" hose on the right-side LP port closest to my back and the primary reg hose on the right-side LP port furthest from my back. YMMV.