For us, a cement mixer with a lid seems to do the trick. 100# fit fine.
How I mix the balanced complete feed: First the premix with salt, a few minerals, spices, vitamins, probiotics, etc., in a carrier (for instance, wheat bran) are put in a large ziploc and then mixed by turning the bag upside down back and forth repeatedly, etc.
The other ingredients are weighed, prepared, etc., and put in the cement mixer.
A hole is dug in the middle of ingredients in the cement mixer and then the premix is placed there and then covered.
The mixer is turned on (timing switch set to 50 minutes) and is tilted at two different angles while mixing.
Since we use quite a few different powdery ingredients (macro and trace minerals, etc., etc.), as well as very minute amounts of some ingredients, I feel that
thorough mixing is critical. This link
http://www.dsm.com/en_US/html/dnpna/anh_swin_mixing.htm and explains that it's a given in livestock operations that thorough mixing of feed is important. This link animalscience.ucdavis.edu/faculty/zinn/pdf/04.pdf explains that the principle objective to mixing feed is to make sure that the animal receives all of its necessary daily nutrients each day. It seems to be pretty standard stuff in feed mixing literature.
When making scratch grain mix (not balanced complete feed), I just toss in scoops of this or that based on my objective for that particular group of chickens. For instance: Three scoops of grain X into the five-gallon bucket, one scoop of grain Y, and half a scoop of grain Z. Swirl it with my hand a few times and call it good. Sometimes I fill the whole five-gallon bucket and then I will layer it so that it's mixed as I scoop it out into the feeder or for tossing.
Balanced complete feed, where each ingredient must be carefully weighed, though is completely different from
scratch grain mix.