One of the many elements a spot shooter must pay attention to is that he or she holds the rifle under a constant cant angle. Variation in the cant angle will cause variation in the point of impact, even with perfectly centered sights and with a perfectly still weapon.
But in what direction does the point of impact shift for a given cant angle fluctuation? And how far? How much margin is available before the effect becomes visible on the target and in the score? Does it matter under which cant angle the rifle was zeroed? Does the use of riser blocks under the sights increase this effect?