[Greenbuilding] rainwater storage & use model

[Reuben Deumling]

I've built a spreadsheet model that uses daily rainfall on site, daily
indoor (municipal) water usage, roof area, and a multiplier (currently 0.7)
for system losses, to estimate what storage volume I'd need to bridge our
90+day summer with little rain. My goal is to use no municipal water at all,
and to have adequate but not excessive storage.

My question is whether anyone on this list has worked on this problem, or
can offer suggestions as to what fraction of rain hitting a certain area of
roof can be assumed to be capturable?

Our rain in Western Oregon is often very gentle--I wouldn't expect to get
much from days when it drizzled. This would suggest using a formula that
only counted rain that fell after, say, 0.05 inches (?) was reached on a
given day.  Based on my calculations so far we aren't going to be short of
water--I'm just interested in not oversizing my tank unnecessarily.

Thanks.

Reuben Deumling