Downrigger technique

Having never fished a downrigger on a PB, I'm feeling my way off the kayak. 

How far back does one trail the gear?  The mainline can be held by the release anywhere above the main swivel.  I could trail 100' of mainline behind the cannonball.   

10-15feet. too long and you won't be able to measure depth accurately, when turning creates a slack, and too far back makes the release clip less functional especially for "shakers". at 10' you can scent your weight instead of your gear

It depends on when/where your fishing.  I picked up a 3 lb Rainbow on Lake Washington this weekend and I was 100+ feet behind the clip at 45'.  These fish are hanging in a very narrow slice of the lake right now, just below the thermocline, so I wasn't too far off.  But for salmon, especially Coho, they will hit it 6 feet behind the lead, so why waste time paying out all that extra line.  For Coho on the sound, a flasher hanging off the lead works leaving you mainline free, this works best if your lure/bait doesn't need additional action - such as a spoon or cut plug bait.

here's an awesome article. it has a lot of info but the lower half should answer a lot!
http://www.squidoo.com/salmon-fishing-in-southeast-alaska
sorry forgot to paste link