Intent in protection spell

I'm ready to make my first practical spell, and it's going to be a protection spell. Everything else is ready, but I'm strugling with The intent. Many people says, that it needs to be precise. How does this work with protection?

For example If The intend is to prevent harm, can it still cause something to not happen that should? Or If you make a home protection spell and you intend is to not let anyone in who has ill intend, can it still cause some good people to not enter, because of one bad thing?

How precise does intend need to be?