Good question. Mostly it is guesswork based on what I saw from the trout last year. Amazingly fast growth on feed, the best I've ever seen. Aggressiveness was part of it, but maybe their digestive system was as well.
Smaller rainbows are probably around 3 or 4 ounces each, so twenty pounds would be 80 to 100 fish. In order to get them to desired size, maybe 16 ounces, and assuming feed conversion rate of 1.4 to 1, that means 105 pounds of feed for 100 fish, about 85 pounds for 80 fish.
Birds and disease will certainly take out some, of course. My forage pond is very small, maybe 1/8 acre, so I can only push it so far before release. Of course, it helps that this will be in the coldest time of the year.
No problem at all with the feed, as I have a TH feeder on site and that much feed would only mean two trips to refill.