I need help to set up the tonearm in my Rega P3-24 TT.

As a first step, I pulled out the anti-skate fully and set the tracking force dial to zero. When I try to balance the counter weight so that the arm floats, I am having a problem. The tonearm keeps moving outward until it hits the tonearm rest and  not staying to place to see if the arm is floating. Could some one let me know why this is happening and what should I do.

I did make sure the table itself is horizontal using a spirit level.

Thanks 

Pradeep