I just changed my 2012 Prius C coolant. Including the radiator, inverter, and reservoir, I only drained about 1 gallon of coolant. Is that the right amount? It is much smaller than the specification of 7.3 qt (5.6 + 1.7), even though some coolant remains in the engine block. During coolant draining, I only jacked up the front of the car while leaving the back wheels on the ground. I don't think that makes much of a difference though.
As you said, some coolant will stay in the block/hoses/heater core but usually not very much stays...A quick test that I would suggest is to add about a quart of distilled water to see how much of it immediately comes out (the full quart should come out). Be sure to measure accurately if the full quart came out. Also jack up the rear a little to create an even drain. When refilling with coolant, don't forget to "burp" the radiator. This is done by removing the radiator cap, running the vehicle for a few minutes and letting the coolant reach +180 F so the thermostat opens up and coolant starts cycling through the system. The radiator will the start relieving some air. Add coolant as needed.