Depends on the sink- is it a pedestal or a countertop sink? If it's a pedestal, you'll need to get the dimensions from the manufacturer (they all post their roughin dimensions on line now). If it's a drop-in, you'll need to figure out the top height, then drop your roughs low enough to get below the bottom of the sink, allow for the trap, etc.- again, having the manufacturer's sheet would help greatly.