I assume this has already been posted several times, but it gets annoying.
Have the option to "FLIP" the constrained value to the other direction. Seems like no matter the order the parts are selected, One will have to
put a negative value in the constraint to make it go in the direction you desire. To avoid this, just type in the value, and then pick a button like in the pattern for the side to offset to.
This way nobody has to worry about the selection order or having negative values in the constraints. It will just flip-flop between directions.