Fox,
I have a very basic KD double flare tool and it works fine. It has serrations in the clamp halves that grip the tubing adequately without any muscle man stuff. It helps to remove the burr left by the tube cutter on the inside of the tube and to put a little oil on the dies to help them form the double flare. Also varying the amount of tube that protrudes above the clamp can help you make the perfect double flare.
