In SI units calculation is pretty straight forward. 1 mm of rain for 1 m2 roof area is 1 liter, and 1000 liters equal to 1 ton of water. 80% efficiency is reasonable for mediterraean climate.
Though there are couple of issues to consider.
Unlike other regions we have a big first flush ecent following the first rains in september. As probably known, first flush brings a lot of debris which have to be diverted. During the 6 month dry period, our roof gets covered with dust, dead criters, leaves etc. I connected 800m2 - ~8000ft2 roof area to the first flush tank (settlement tank) (2m3 ~ 2 cubic
yard), it fills up with debris after september rains. Thus it is better to clean tanks after 2 - 3 weeks of "first" rains (september rains).
Rain water is gray water, meaning that it is not clean enough to store in a tank. Processing rain water to store is expensive. It just doesn't cut the costs. Storing rain water in soil is one option, storing in a pond/ lake is another. I chose to store in soil. Rain water travels to the main collection tank after the settlement tank. Water does not stay more then couple of days. Sprinkles kick in 2 days after the rain event and if there is more water incoming then the capacity of the collection tank (30m3) another
pump pumps the excess to swales.
I used to spend 10 cubic meter of water per day on average. 4 months equals to 1200m3 of water to be stored. Those tanks will be expensive for sure. This year the average droped to 1.2 m3.
Solution I came up with: store water in soil, increase water holding capacity of the soil by increasing organic matter,
mulch mulch mulch, shade shade shade.
My stiuation doesn't correspond to yours, but I thought it might give an idea or two. Hope it helps