Sorry, some people say total system volumes but that can get pretty meaningless if you start adding separate tanks for seep sand beds, refugiums, etc. so I usually go by display tank volumes.
To calculate return pump flow rates it might be a good idea to start looking for skimmers to get an idea how much flow-thru the skimmer will require. That will give you an idea of how much you will then need to pump back up to the tank, i.e. the amount that flows down is controlled by how much is pumped back up.
When pumping from a long way away, and especially when pumping from a lower floor up to the display tank, you need to make sure you choose a return pump that not only has the ability to pump a lot of water, but also has the ability to pump against gravity's pressure plus the added resistance of the plumbing, including any turns and valves. This resistance to flow is called "head pressure" and is a separate measurement given by pump manufacturers. To accurately calculate head you must measure the layout of your return plumbing, including all twists, turns and bends, as these all add to the resistance (head) against the return flow. Once you have figured out your plumbing, there are many calculators on the web, including here on RC, for calculating total head pressure. With that information you can begin to shop for pumps, including looking at watts of power used in order to figure out which would be least expensive energy-wise to run.
It doesn't sound like you are reading any books on setting up a reef tank. I would suggest you do so, as all these basic questions are answered there. If you haven't already got any favourite reef authors I would suggest the e-books by
Mr. Saltwater Tank to get you started. His are the most up to date I have seen and he generally agrees with me so I think that makes him okay reading.
Dave.M