Need some suspension/chassis guidance

So let me ask what might be a stupid question. But once I move my springs and do my mini tubs, how do I know if I need to narrow the rear. Obviously if I had wheels already I could just measure. If I put the tires where I want I will probably get a good idea. I realize the wheels will bring the tire bead in a bit but will it give me a good enough idea of where I need to go. I'm going to run a 15x12 rim.