The superbraid mainlines such as Power Pro or Tuf line actually float awhile, and most mono slowly sinks - that's why float fishing for Kings with the braids is one of it's advantages. But it's the much thinner diameter of these braids that get the divers down deeper. Same when using them with a 40" mono leader for plugs; good for deep holes.
As for those huge Jumbo Jet Planer divers, Luhr Jensen rates them a bit optimistically as being able to dive up to 50' deep. I don't think that's easy in most practical applications. But 35' is plenty attainable. When they first came out we used those in the Willamette at Oregon City to backtroll bait for clients that were backbouncing 'challenged'. They were and are a real benefit to the guides. Geeez, you didn't ask for all that. You want to know how far to let out. ~~~
There are too many variables with reel widths, line diameters, and current strength/backtroll rates to suggest a set formula of reel bars to let out. Those Jumbos will usually dive your line at about a 40 degree angle with between 30' to 60' of line out - any more out and the line angle reduces a bit (shallower per amount of line out). So if you have 50' out at a 40 degree angle your depth will be about 60% of that - around 30' deep with 20 lb. mono, if the rodtips are kept relatively low and you use about a 15" diver dropper line. (About 35' deep with thin braid mainline). From my experience at O.C. in the past, we would often 'dredge' bottom too much in 25' deep water during an average backtroll speed with about 50' of line let out, using 20 lb. mono with the Jumbos. Those suckers do dive!!! As I mentioned, there are lots of variables, so if you start bumping bottom bring in some line. If you aren't touching bottom at all then let a little more out. Then stabilize it just off bottom the best you can. ... The 20' and 30' Jet Planer diver line angles are usually around 30 -/+ degrees. So up to 20' depths the '20' JP will commonly be a little less than half as deep as the amount of line out, while the '30' JP will be close to half as deep, up to 60' out. In all cases with these divers, sometimes the lesser currents along the bottom which varies with the amount of boulders etc., you may have to let a little more line out than those guidelines to get it near bottom. ~
I suggest establishing the depth of the water you are going to be backtrolling (or anchor fishing) with your fish finder, then backtroll it at appropriate speed and see how much line out it takes to bump bottom. Do this by allowing it to stablize at a certain length out for a bit, then keeping your rodtip low, pull it upstream a few feet to see if it bumps bottom. If not, try it further out until it does, then bring in just a little, and record what length line you have out for that depth and water type. Then you will have a handle on about how deep you will be at particular 'bars' of line (length) let out. ~
Since most of the bites will come near bottom (around 2 or 3 feet off is good to try) you should fish your bait or Kwikies deep. Just remember that the Kwikies also dive, so keep your diver a little further off bottom for those than for bait. Also, have one of your crew try fishing several feet off bottom for suspended fish, which occurs more often than some think it does. ~
For springers, I prefer to backbounce bait near bottom with a 22" to 30" lead dropper to bottom bounce in deep Columbia water; 18" to 26" in Willamette holes - both depending on the conditions and baits used. Although last year in the lower drought water, backbouncing eggs below the Bonneville deadline for them we found that out near the middle they would often really hug bottom and prefer the eggs right there - thus using shorter 1' droppers and laying the bait right down without jig tapping the lead would often get a soft and sure pickup from the springers. But that was an unusual spring for sure; and doing that can waste a lot of eggs to the squawfish in places. When using a lead dropper to fish Kwikies deep I only use the lead to touch bottom occassionally, keeping it up off bottom a couple cranks or so; because they dive several inches on a 45" to 50" leader. ... btw, using lures such as Alvins for Columbia fall chinook in warm water, fishing 35' to 40' deep with up to 4' droppers seems to draw more strikes than the shorter droppers of spring.
Hope the long ramble helps. (these techs are from my manuscript).