Flavoring hops are generally added from 30 min to 15 min of the boil. Aroma hops from 15 min to 0 min. To balance bitterness and flavor, start to look at the ratio of hops used through out the process. AA% play the role of bitterness and can be calculated, but flavor and aroma do not depend on AA%, they come from the type of hop and quantity used. Reference other recipes, and experiment, experiment, experiment. Hop storage conditions also play a role, as old stale hops will not give much flavor or aroma. It gets much more complex than this, but its a start.