Another way to avoid the problem is to use a small A-frame in front of the anchor stone to change the direction of the rope so that it is pulling vertically on the stone. Since the tension in both parts of the rope are roughly the same, the A-frame needs to incline at about 45 degrees towards the anchor stone. This would require some shallow, angled holes to prevent it skidding. They don't need to be very deep, 6" or so should be OK. The A-frame also has the effect of increasing the height of the rope which gives a better range of movement to the weighting stone and might mean that we could do the whole job in a single "raise and lower" sequence.