From the book:

f. Marked-Sling Method. To use this method, the grenadier must--

(1) Loosen the sling, assume a kneeling position, and place the forward foot in the sling.

(2) Ensure the sling is taut and vertical between the front sling swivel and the boot. If not, the rounds will impact at a greater range than desired. To check this, tie one end of a string or thread to a weight, such as a cartridge case, and tie the other to the sling swivel. Let it hang freely, and align the edge of the sling with it to ensure the sling is vertical.

(3) Fire several rounds to determine the desired range.

(4) Where the sling is held to the ground by the foot, mark the sling with colored tape, paint, ink, or whatever is available. Mark the position of the buckles so that, if either is moved, the grenadier can return them to their original positions and be assured of constant range accuracy.

(5) If the sling gets wet, it may stretch or shrink, indirectly causing the rounds to impact closer or farther than desired.

LINK.