I live in West Seattle and I'm an accomplished DIY. I'm thinking about putting an irrigation system in my yard so I can water my grass. I would prefer not to use city water to do this so I'm leaning towards using an underground spherical holding tank that would collect my roof's rain runoff. I'm not sure on how big of a tank I would need or how I would hook up my sprinkler system to it?

I've researched NORWESCO tanks and the 300 gallon under ground tank with pump platform looks like the solution but I'm not sure. My roof pitch is 6/12 and its square footage is around 2500. My lot size is around 5000 square feet and maybe 2300 of that would be considered my yard. Would a 300 gallon tank do the job? Would Seattle's rain be able to fill up the tank so I would have enough during Spring and Summer? If so, how do I hook up the sprinkler system to it so the pump can provide the lines with water?