So when considering that 15 out of 36 attempts by the attacker will be successful and 21 will be won by the defender we need to balance the odds. To get to our magic 50% we need 18 of our 36 rolls to be winners! Thus we need 3 more rolls than we are currently achieving, 3/15 gives us the extra number of rolls we require to win, a nice 0.2 which means I need 1.2 dice to be on equal terms with the defender.
Aha "you can't have 0.2 of a dice" I hear you say, which sadly is true so all in all I've wasted 2 minutes typing this up! Stick to the rule of two to one when attacking and you'll be dandy although the above doesn't account for SOD's law which appears to apply to me in this game more than anyone else. Damn you Mike-"Genghis"-Croft!