Low flow toilets / Rainwater Harvesting Manual

I've been looking at low flow toilets. The best ones are pressurized and have dual flush (3 or 6 liter/.8 or 1.6 gal) I figure if we were to use a rainwater collection system we'd need the following. 1.6 gpf x 60 users x 3 uses per day x 3 days (length of most gatherings) = 864 gallons of water needed. Hopefully, we could reduce that number by getting folks to use the composting toilets as often as possible and use a half flush when possible. And I see these numbers on the high end. It assumes no one uses the other toilets on the land. None of the pressurized toilets I've seen need electricity. It's a small pressurized tank within the toilet itself. I've been looking online for descriptions of how to design a rainwater collection system. The best I've found so far is from Texas. The Texas Manual on Rainwater Harvesting by the Texas Water Board. It has examples of many simple systems which we could use as well as tables for figuring use needs.