If you can't get the return to happen by dragging the maneuver node around, remember that the farther away from Kerbin you make your correction burn the less fuel you need for it. So in case you still need to do it, it's best done right after you leave Mun's SOI. Personally, my first time visiting Mun I had to wait a quarter month before returning because I didn't pay attention and took off into a near polar orbit. I didn't have the skill to correct inclination nor enough fuel to brute force my way home, so my only option was to wait until the Munar cycle brought Kerbin into my orbital plane. Thanks to time acceleration that wasn't as painful or boring (to me, poor Bob might disagree) as it may sound.