I'd attach the column and steer all the way to the left and then back the other way and count the turns. Then count back exactly half the amount and that will put it very close to centre enough for you to set the pitman arm in place.
Odd question should I have the box hooked to the pump and running while figuring center? I didn't know if pump pressure makes a difference while doing this. Box still has the plastic plugs in it I haven't hooked the new hoses up to it yet.