Table: Missile Fire for Large-Scale Combat

The table below should be used for ranged attacks by troops, excluding gunpowder, catapult or magical attacks (i.e. it only applies to groups of soldiers attack with bows and crossbows): 

From Chainmail, page 11.

NOTE: Cover. Soft cover such as brush, woods, waist-high fences, and walls reduces missile casualties by one-half (drop all fractions). Woods and overhead cover, such as roofs, prevent all indirect fire. If indirect fire is possible, casualties are not reduced by cover. Movable mantlets and walls with arrow slits make targets behind them, including missile troops firing through such slits, “arrow-proof,” unless indirect fire is possible.