If both ends of the hose/pipe are submerged, the calculation is water level to water level. For canisters this means that the head loss is actually ZERO since both the suction & return lines are submerged and the canister itself is sealed.
The pump end is always calculated from the water level of it's container (pumps/intakes are always submerged, obviously). If the other end is not submerged than the calculation is to the end of the hose/pipe.
>> Jim C. <<