Just to quibble, when I divide 5 with 1200 I come up with 0.0041666666, not 0.42. However, I will be the VERY first to admit my math skills are less then stellar and I may be misremembering long ago math classes on how to present this number.
However, that is pointless since we both are making the point that the "Insurgency" is giving us another concrete example of their growing irrelevancy to what is happening in Iraq.
Correct: 5/1200 is 0.00416666. However, when expressing it as a percentage, as you originally did, you multiply by 100 which moves the decimal place to the right two digits. I merely rounded to 0.42%, which is common, IMHO.
Nevertheless, we are in full agreement on the point itself: that the ratio is disheartening to the terrorists and the Democrats.