Hi Goliathus,
I use 1lb of wheat bran for every 5 lbs of raw pellets. So 4 lbs for a 20 lb bag of pellets. So I guess that is 16.66 percent of the total. I honestly don't know if thats the right ratio because I haven't really tried other amounts, just always did it that way. Like you have mentioned, it seems like the heat is the key factor. If its really hot, it turns black in a month or so but if its not that hot, it only gets dark brown. So for me, it seems that temperature is more the key. I used to ferment twice. First the 1 lb per 5 lbs of pellets and then two months later, another 1/2 lb of wheat for 5 lbs of pellets and then it would get pretty dark after the 2nd time. Now I just do it once when its hot and seems be better than doing it twice when its cooler.