You essentially want your crit to scale from 0% to 100% with diminishing returns. Here is a formula that is 0% at luck=0 and approaches 100% as luck goes to infinity.
f = 2.71; // you can mess with this factor to change how quickly it diminishes
crit = Math.random() > Math.pow(f, -luck); // assuming random is in the range [0.0, 1.0]
Then use a multiplier if there was a crit
mult = 2; // set for a multiplier of 2
Then calculate the base power of the attack. I just set it to strength + 10
base_power = strength + 10;
Then just use the multiplier if crit was true
damage = base_power * (crit ? mult : 1);
Let me know if you need something different, and I can update my answer.