I down rig fish 90% of the time and use a Scotty Depthmaster....I love this product.
It's mounted on the L/H side of the side board .....at the end, closest to me and my rod holder is about 7" forward using a Scotty flush mount as shown in the photo below. The Scotty power lock rod holder with the gear head mount extender are solid products and make for easy adjustment of your rod position.....no knee clearance issues when peddling the PA.
I also cut the arm down to 12" long after using it and found no real advantage with it farther out from the side of my PA-14...... a also re-spooled the drum with 80 pound test Spyderwire, stealth braid to eliminate cable hum.
My advise is to keep the down digger close to you for ease of deployment and recovery of your weight....I use my left hand to deploy/release the weight and my right hand thumb on my reel spool as the lure is deployed. It's simple and works very well for my use.
I added a "hook to the arm" in order to hang my weight while I connected my line to the release clip.