I always use 3 cards and get a much simpler answer, which is what you are looking for in a "yes/no" situation - the answer without the smaller details. The center card is always my "start" point for any 3 or 5-card spread, so usually whatever this card is will give me the bulk of the answer, with the two on either side supporting this answer.
I also try to avoid questions that start out with "should I" as a yes or no answer, because I don't see the Lenormand cards caring what you should do, only what you WILL do and what will happen. When you do start them off as "should I", it's more like asking what the outcome will be if you do said action.
For example, I just asked my cards "Should I help my other half find a job?" and since I planned to, it's more looking at the outcome of my help. Key-Birch-Letter comes up, telling me he may be successful if I help, but it will probably cause an argument. Now it's up to me to decide what to do, without the cards actually giving a straight forward answer.
Hope this makes sense!