Any empty squares on a partially completed line that are too distant to form part of any of the blocks described in the clue can be determined to be definitely empty and marked with Xs.

📝 Example: on a 15x15 grid, if a line has a clue of "5" and only the middle square is filled, we can deduce that any square further than four squares away from that filled square must be blank. This would leave four 'unknown' squares on each side of the filled square, accommodating all five possible positions in which the block can be placed.