Combined Scale
A handy way to keep track of where the party members are in relation to each other is to have them set up their marching order in melee scale. Set up the monster group in melee scale in relation to each other. Now, assume that all the spaces between the party and the monsters are missile squares. Make sure you indicate where the scale changes by setting a pencil or ruler on the map.
This combined scale method assures that monsters trying to close under missile fire have to withstand the normal amount of attacks before they get close enough for melee. It’s also useful for determing the areas of effect of any spells cast at the opposing group. When any two opposing characters come in contact, switch the scale.