I've always done what you've done, put the car on an incline with nose up. Filled it up, leave cap off. start motor, when it gets hot I watch the coolant go down (thermostat is now open and cooling motor, if this doesn't happen, you have a bad thermostat) and then I fill it back up, repeat until it overflows. when it starts overflowing I put the cap back on then make sure the reservoir is full.