It's more than just cable tension. It's the amount of cable pull. Standard Avid mechanical disk calipers are meant to be used with V-brake levers.
Avid makes calipers designed specifically for road levers, which don't pull as much as a MTB V-brake lever.
You can buy a little gizmo called a travel agent, which will go really far in improving stopping power. Additionally, you may need to adjust the rear brakes a bit. Here is the basic rundown.
1)Loosen the two bolts holding the caliper to the frame
2)Crank in the big red knobs on the sides of the caliper until the pads are sqeezing the rotor, holding the caliper in place.
3)moving from one bolt to the next, tighten each of the two bolts holding the caliper to the frame a little at a time until fully tight.
4)Back off the inside red knob *just* enough that the pad clears the rotor.
5)Back off the outside knob just enough to clear the rotor, and play with cable tension to adjust feel.