Resetting A-axis

I did some manual G-code to use a 4th axis rotary basically like a lathe.  The A axis doesn't have a homing switch.  It advances a specified number of turns as the X and Y does its thing.  

Part gets changed out, repeat.  

What's the correct G-code to reset the A axis?  Numerically, it's spooled up like 1000 turns in one direction.  Moves aren't relative and I don't need the thing to unspool 1000 turns to get back to the absolute A-axis value the next G-code run will start at.