The Only Way To Tell Whether Something Is real Is To See If It Works For You. Is This A Legitimate Principle?

I don't think so, if speaking to physical reality, however, if speaking to feelings or modesty than yes it might be a legitimate principal, when you experience events through your life, your reality becomes different by the second than everyone else. The fact that everyone that lives in this world called "Earth" has a different reality is the main reason why your reality becomes real and different to you, per say, when everyone plays a different view of things upon the reality of events such as physical events (Laws of Physics, Matter, Energy..), social events (Relationships, Friendships, Social Interactions,...), human feelings and emotions (Breakups, Lost of someone loved, Fights, Anger, Happiness, Love, Affection, Contempt, depression, Anxiety, Desire, Despair, Disappointment, Ecstasy, Frustration, Boredom...) anything based upon this subjects that make up yours and everyone else's reality.

The most basic understanding of what really stands around you are the none social events, such as physical things, like when light reflects upon an object such as a tree and your eyes indicate that the tree is physically there. In fact, that particular tree that your eyes are seeing will form part of everyone's reality and your reality as well, when that particular tree is there. Using your senses to detect what physical object is in front of you. However, if your own reality of things are based upon who you are, what have you done, what is your position in your society, etc...than everyone's reality becomes different but not necessarily has to work for you or anyone else in order to be real.

To some people when something is real is when feelings and emotions such as love, a love that can or must be mutual in order to know if what they want for their reality works for both of them or not. The same way if something is real to you but not to her, and in the same way if it works for you but not to him, than that kind of reality  makes it different for both of them because they want or don't want  different aspects of what reality of relationship they want to have.

Reality in my opinion can be based in different categories like mentioned before, the reality of the physical, the reality of the world, the reality of emotions and feelings, the reality of society. They are all based upon events that makes it different to everyone else, this functions properly like a perpetual motion machine when all of the aspects of reality work together.

If something is real because it works to you and makes you alive, than yes, in order for something to be real must work for you, (Speaking) for the aspect of every single individual. However, if is speaking in the physical aspects of nature, than no, not necessarily. A tree is in your reality because is there and even if it doesn't work for you that the presence of the tree is there, you cannot make a tree disappear with your thoughts so it can live this world just to make it work for your reality in the real world we live in. If you think the opposite nevertheless, than I'm sorry friend but the world doesn't revolve around you, Copernicus taught us  "The Earth revolves around the Sun, not the Sun revolving around the Earth".