A bit of a dead thread, but since it has been resurrected I shall submit the following...
Way back when I first started learning how to bend sheet metal, I was taught a somewhat "rule of thumb" method of using set backs. The person who taught me this, although a real bastard to deal with, knew his fabrication skills very well. I credit him for some of the skills I retain to this day. I am also plenty happy to be rid of him.
But there I go digressing again....
Attached is a chart that I derived beginning with his memorized values for setback in bending steel sheet. From those, I was able to come up with a simple formula that gets you very close without the complexity of the K-factor method. The formula used in the chart is: SETBACK = (2 * THICKNESS) - SHRINKAGE
Where SHRINKAGE = 1/3 * THICKNESS
It has been quite a while since I used this, as I just use SolidWORKS now to develop a blank for bending, but if I recall correctly the blank length is made up by unfolding all of your true lengths then subtracting ONE SETBACK for each bend from the total bend length. In other words, you SET BACK each bend by the amount in the chart.
Attached image and spreadsheet versions of the aforementioned chart.