[scuba1000]

When constraining I usually use the rotate tool to get the part as close as possible to it's final position, then I constrain. Also, if it flips over, you can change the option in the constrain window.

I don't fully understand what you are saying, but it sound's like using the offset will do what you want to do. Constrain the two planes so they are touching and then offset it, either neg or pos) the distance you want to move it. The planes will be parallel but not touching.