Also, since we are going slower, I'm not sure we need an 8lb ball, has anyone tried a lighter one?
I thought the same thing when I started with my 2.5 kg (5.5 lbs) so I tried a section of stainless steel rod that weighed in at 982 grams (2.16 lbs) and found it way to light, became to much of a guessing game as to true running depth which defeated the purpose (for me) of fitting the downrigger in the first place.
Then got my hands on some scrap steel rod to try, it weighed in at 1.6 kg (3.5 lbs) and was still unhappy with the result, it was just ok at around the 2 to 3 knot range but once I got above 3 and up to the 5 knot range (preferred speeds) it didn't cut the mustard. I find I can judge the 2.5 kg (5.5 lbs) pretty good in 21 meters (69 feet) and doubt my home made downrigger break could handle much more weight.
I plan on trying a spreader bar on one of my two 2.5 kg (5.5) balls so I can run two lures at once, but I am guessing the extra drag of the bar will make it harder again to judge true depth and may be the bag of sand that breaks the camels back as far as my downrigger break is concerned. Time will tell.
So how far behind is it traveling, well I have my transducer mounted off my rudder, I work on the idea that for every 1 meter of depth I have 1 meter of floor coverage (cone) from my sounder, the downrigger is about 1.75 meters in front of the transducer and in 21 meters I loose sight of the downdrigger line about the 16 meter mark (depending on speed) I am hopeless at math so jump in and correct me if I am wrong;
first halve the cone size, 21/2 = 10.5
add distance from trany to downrigger, 10.5+1.75 = 12.25
rate of curve per meter, 16/12.25 = 1.3
distance behind in 21 meters, 1.3x21 27 meters
seems a lot when it's in writing
I like using Rapala magnum in size 20 & 30, & deep divers in TDD11 TDD09 set my weight at about the 16 meter mark (so I can see it) and adjust line length so my lures are at around 20 meters so I don't spook the fish with the ball.
The deeper you go, you probably need a bigger ball, and if you are like cliffs2yak and fishing in 150' to 300' with all that extra line drag, then I would say you may need that 8 pounder (glad it's you winding it back up)