Another very accurate way to cut slots in KS square brass tubing is to use a drill press...Just chuck in a large fiberglass cutting wheel. Then scribe 2 lines equal to the width you want the slot to be centered on the tube.

Carefully aline one line to the outside of the cutoff wheel with the tube flush on the drill press table. Make a couple of passes by carefully sliding the tube against the cut off wheel, then turn the tube over and repeat the passes. By making shallow cuts, gradually piercing the tube you will reduce the chance of the wheel grabbing the tube.

The process is much easier than it is to describe. You will have a perfectly parellel slot the exact width you want. I usually fold over a small piece of 320 grit wet paper & burnish the slot smooth.
Bill