Suggestion; if you're going through 15# of dry feed a month with free access before FF, you can figure to go through no MORE than that fermented. Calculate out your daily dry feed needs (sounds like about 0.5lbs/day?) and ferment and ration accordingly. So if you wanna do a fresh fermentation to feed daily, ferment .5lbs every day, if you wanna do a weeks worth, ferment 3.5lbs and feed out through the week. If they finish that every day and don't get super fat you're not doing them any harm and you can creep down from there as-needed to encourage more free ranging or up to make up for a lack of forage in winter. I'd say however much dry they were eating before FF is a good expectation for the tops of how much dry matter you should ferment for them.