Using Stepper Motor like Hobbyist Servo
You would need no install a referennce sensor and do the homing procedure each start up. A ref. sensor can be a mechanical or inductive switch located possibly at 0 position.
At start up you begin to move the stepper motor slowly bkw (if the signal isn't present), after the switch is detected you stop and move very slow fwd until the switch looses again. At this point you store the homing position, for ex. 5 or 10 degrees. Then you do movement, you can go to 0 or wherever you want.  

EDIT:
One another possibilty is to go slowly bkw for the time the throttle needs to retract from max open position, the motor will come to the hard endstop (you need to provide a hard stop without damage). The motor will stall all the time it will be forced to move further backward. Then after predetermined time you stop and store 0 (or -5, yet better) point.