There are two considerations: force and travel. You need to make sure the strut, and the mounting, allow the head to move full travel, so the travel of the strut needs to be equal to, or exceed, the travel of the head. Then, the force needs to be less than the weight of the head. How much less is open to debate. I would suggest making the spring force no more than half the weight of the head. You still need enough weight for the head to settle to the bottom of any backlash on it's own, after being moved down. The weight of the X2 head is quite small - even with the motor I doubt it weighs more than 30-40 pounds.