RMB the view border.
Select “Active Sketch View”
Draw section line with dimension (42mm)
Select “Section Line” command.
Under "Definition= Select Sketch", Select the above drawn section line and press OK.
Click Section view command and select “Definition = Select Existing” and select the drawn section line.
Place the section view.
Double click the drawn section line. Under Definition select sketch Icon. Edit the dimension (42mm)
Finish the sketch and then exit the Section Line window.
Rebuild the section view to see the new section view.
I've attached a possible solution for NX9. It involves using the Offset Option on the Point command when selecting an object to inter point. It also requires that you have a curve (or line) perpendicular to your Datum A, though you could use the remaining offset options to see if they provide a solution.
I had tried this a few times, but couldn't get the offset to work. I was using a line for the offest vector, but that line was in a separate view. No matter what offset I entered, it would put it on the datum. Since I am working with a cast part, every wall in that direction is drafted. If I picked the drafted line, my dimension would be just a hair off. I ended up using the drafted wall as my line and kept fudging the number just a little until the dimension measured 42.00. Thanks for the help!!