Centering a Square Workpiece in a 4-Jaw Chuck

I should have looked this up before spending a lot of time trying to do it.  The answer is here: center a round piece of stock the same size as the edge of the square piece (which is pretty easy), then replace the round piece with the square workpiece.

I am working on the square tubing to hold round stock securely in position to the fence on a chop saw.  I am glad that I went ahead and built a sample using 1.5" x 1.5" tube first.  I planned to use a 6" long square tube to hold the round workpiece in position.  Because the interior dimensions of a 1.5" x 1.5" x .125" wall square tube are actually slightly smaller than the nominal 1.25", I knew that I was going to have to bore the interior of the square tube slightly so that the round workpiece would slide through.  But I completely forgot that to use the lathe to bore the entire 6" length of the square tube is not possible: my boring bar isn't long enough.  (Does that sound like the subject line on a piece of spam?)  Instead, I have cut the 6" piece into two 3" pieces, and bored the interior from each end of each piece. 

Now I will use a long piece of material in conjunction with the workpiece clamp on the chop saw to hold the round workpiece in position on the left side of the blade, and the same approach on the right side.  (Although on the right side, I will only bore the square tube 1" deep.  This gives me a consistent stop so that I don't have to remeasure each time I cut and advance the workpiece.)