This cream is a keratolytic product: its acids are meant to loosen thickened layers of dead skin. Unaffected skin already renews itself normally, so applying a keratolytic acid there does not give any benefit. Instead, it can over-soften or irritate the skin, especially in areas where the skin is thinner, such as the top of the foot or between the toes.
Hard skin usually appears in specific places: the heels, the ball of the foot, the sides of the big toe, or wherever shoes and walking create pressure. The cream should be aimed exactly at those spots. This is not just about using less product; it is about preventing contact with skin that does not need this type of action.
In practice, apply a small amount directly to the hardened patch and try to keep it within that area. If you find it difficult to be precise, apply it in the evening so it can absorb while you are not walking, and do not spread it far beyond the rough spot. Avoid the instep, the spaces between toes, and any unaffected skin next to the callus.
Using less over a smaller area is also a sensible way to see how your skin reacts before you use it more regularly. If irritation appears, stop using it and wash the area with water.