Sump for a 29 gallon

Hey y'all sump question. Petco's $1 a gallon sale has got me thinking about building a sump for my 29 gallon. From the research I have been doing it seams that a 10 gallon tank would be sufficient for my DT size. I'm thinking of a rather simple setup, over flow into a filter sock, through a baffle into a second chamber with the skimmer, heater, maybe a chunk or two of Live Rock, then through a bubble trap to the return pump. My question is, how would I go about figuring out in the event of a power outage how much water I can expect to flow down to the sump? is this based purely on where the overflow site? My tank is tempered so I cant drill it and will have to use an external over flow. Sorry if this is a n00b topic!