Most wargames and a few RPG games use a scatter diagram and relative degree of success to represent indirect fire.
You roll to see if you hit a designated area. If you miss you still hit an area but at some distance from the target. You roll to see if you hit armor class 10 modified by bow's range. If you can't see the target you are at an additional -3. For every one you miss by you shift one hex or square.
If you are using squares then roll a d8 to see which direction you shift.
If there is a target in the square Then make another to hit roll. You are looking to beat the person's AC or roll a natural 20. This represents the chance whether the arrow penetrates the armor and is basically pure blind chance. If the arrow magical then apply that bonus to this roll.
If you are using Ascending AC and the to hit bonus for levels then this is a straight roll against the target's AC.
If you are using Descending AC and the level charts you roll as if you are a o-level human against the target's AC.
The chances stink if you are single archer trying to use indirect fire. However if you have some retainers and the party coordinates their bow fire the result can be devastating if target are otherwise hard to get at.
Again the first roll is to see whether you hit the target area. The second whether the arrow penetrates the armor.