Cistern Capacity

I'm looking at the LEED-2009 WEc2 LEED Online form, and it asks for "annual quantities" of captured rainwater, recycled graywater, etc. So it seems to be looking for a quantity of water, not a cistern size. Which makes sense, because LEED wants to know how much potable water you're replacing—the system capacity you use to do it is of less interest.

Tristan, you are correct on that, but in regards to this question, If the cistern does not have the capacity to capture all this rainwater, the rest of the water is going to be lost and not able to replace some of the potable water for the flush systems. In our case we have an annual rainwater of 869kGal, but the cistern capacity is around 17kGal. Therefore tecnically the annual "captured" rainwater should be calculated in some type of spreadsheet based on the capacity, is that correct? If that's the case, does anybody know of a spreadsheet where we can calculate this?

Victor, it sounds like what you need is the help of an engineer who has developed an appropriate calculation method. I'm not aware of a spreadsheet out there that can do this for you—or if there is one, I would imagine that it requires some expertise to use.