Turn the steering wheel lock to lock without the drag link connected. Count the turns both ways so that you can figure out where center is. Make sure that when the wheel is centered, the pitman arm is straight. If it's not, pop it off and move it over a tooth or three until it is.
Make sure the tires are straight, then measure from the pitman arm hole to the hole on the knuckle or wherever the drag link mounts. That's your total drag link length. Subtract out the exposed length of the rod ends (not counting the threads), plus a little extra so there's some adjustability.
