Actually, explaining how firing missiles and spells functions happens to be one point in which the manual exceeds very well.
In general: the higher your "precision" value, the better chances you have of hitting targets with ranged spells or missiles. Boosting your precision value (with items or with spells) will have a noticable effect.
Getting closer helps too!
Cherry Saber made a great thread here which may help you further.
http://www.shrapnelgames.com/cgi-bin...;f=74;t=000172