The reason stock tanks look like they will hold more than you can put in them is because of temperature expansion. They allow extra volume so that when the fuel expands as it warms and evaporates, it has room. If you don't allow for this, the fuel will find somewhere to go. carb, charcoal cannister, out the cap (the non sealed ones), or could possibly push a return line off somewhere. 10% of the volume should be more than plenty. and remember a vent line (from the highest point in the tank) to gather and route the vapors to the charcoal cannister.
