I just came across your post on BYC and followed the link there. I did this several months ago in one of my coops (I have two coops) with nine girls in the coop where I installed the feeder. I have to say it works GREAT. I wasn't aware that there was a 3" drain pipe when I built mine, I used 4" drain pipe and my numbers were extremely close to yours, but I bought a 12' piece of the pipe and intended to make a second feeder for the other coop (eight girls and a rooster). I realized today that I don't have the 90 degree elbow, nor the two caps, so the next time I go to town those are on my shopping list.
I cut the 12' piece of pipe in half, then cut one piece into two pieces, one is 2 feet and the other four feet, then I cut two inch holes, and the girls dip their heads to get the feed. I think it works wonderfully, and I only have to refill it once a week with nine girls.
I am still using the older type metal feeder with the ring around the bottom for the second coop, and today I noticed that it was empty (again) after only three days. DARN, all the neighborhood birds are getting into the coop and stealing the feed, so I guess it really IS time to go shopping and get that elbow and two caps, that's all I am lacking to finish this little project. One thing more. I had a really tough time getting the top cap off to refill the food, but then my better half said something about "sanding the interior of the cap to widen it," and I spent an hour sanding that bugger, but you know what ... it worked, easy peasy now to pull that cap off, and at night I just shine a flash light down to see the level of feed, I can see the feed through the thin walls of the 4" drain pipe.
Skip