My birds get FF as 100% of their formulated diet. The general rule of thumb is that a LF hen will eat .25 - .33# of feed/day. You can use that as a guide to start fermenting. It's not rocket science, and if you don't make enough, you can just give them some more dry feed to make up the difference. If you make too much, save the extra for the next day. I use a 5 gal bucket. Fill the bucket 1/2 way with water. Add dry feed, until the resulting mix is thick enough that your stirring spoon will stand up. Think: soft serve ice cream, thick cooked oatmeal, mashed potato consistency! Leave it open to the air, and stir several times/day. If you must cover it, use a linen dish towel or some cheese cloth. But it needs contact with the air so it can get "seeded" with the beneficial yeast and bacteria. When it's bubbling, or puffed up, it's ready to serve. Always hold back a bit to "seed" the next bucket full. After you get a strong culture going, you can feed the birds in the morning, replenish the bucket, and it will be ready to go the next day. Once you get a strong ferment culture going, all you have to do is mix it up every morning, and feed it out the next day. NO further maintenance required. I loosely cover my bucket with a lid, but wouldn't do so with a new culture.
Note the dry feed wt. /bird/day. My FF feed conversion rate varies between .18 - .22#/bird/day.