At 80% of annual rainfall, no supplemental water was required with a 2000 gallon tank and a monthly usage amount of 1125 gallons. I created a calculator to determine what using a 1125 gallons for two people looked like.
I’m planning to use a composting toilet so there’s no category for toilet water usage. The 4 minute shower length might be a little austere but it seems doable. The shower head flow rate really matters. Changing to a really low flow head that delivers 1.6 gallons per minute would increase average shower length to 5 minutes.