We can not justify a physical sin by mentally distracting ourselves from our actions. It's like asking if it's okay to shoplift a CD as long as you are thinking about feeding the homeless. Your mind may be doing one thing, but your actions say another. You could even compare it to the kiss of Judas. His body showed love to Christ, but his intentions were full of sin. When you masturbate while trying not to lust, it's kind of like doing the reverse. You are supposedly doing something noble with your mind, and something disordered with the gift of your sexuality.
The problem with masturbation is not simply the lust of the imagination, but the impurity of the body. But click here for an explanation of that.
What's going on is that you're trying to bargain with the devil. Theology and scripture tells us that Satan is an exceptionally cunning fallen angel. The name "Lucifer" means "light-bearer," and all of this means that his intellectual abilities far surpass anyone on the planet. So, when you try to cut a deal with him, you lose . . . every time. How many times has he duped us into thinking, "Oh, it's not that big of a sin. God will understand. I can go to confession. How am I supposed to be perfect? It will just be this one time. Etc."? We should not reason with our lust. We must rebuke it. So click here for tips on how to overcome the habit.