An easy way to balance your quests is to create the map adding easy, normal and hard difficulty rooms.
First draw the map, then put special rooms with the special monsters, characters or artifacts to build the story. We will call them 'the objective room' of the Quest.
After that and only that,stock the dungeon with creatures by choosing how many easy/normal/hard rooms you want to put in. A monster table categorizing monsters value could help here (goblins are weak, whereas Chaos warrior are difficult to beat).
At the end only put on the secret doors, traps, etc.
Finaly, as you play the Quest, if the heroes are very well equiped (with full armors and weapons), and you feel that your quest is easy for them, try to add more monsters as the game is running. One or two more monsters by room will be fine.