Can you over do a spell?

The common definition of magic, being "The art and science of causing change by means of exerting one's will" should answer this question. Because a focused spell is empowered by your own will, it is difficult to "overdo" it. It will manifest as dictated by your willpower. When working with other entities, however, it is important to be specific in the expression of your desire, and not leave room for mischief.