Don't forget to use all the parts of your body to gain momentum. Just because your other arm isn't doing anything doesn't mean that it can't be moved to change your position and give you more leverage.
Also, when shook says that 2/3 to 3/4 of the maximum extension of your knee is optimum at contact, make sure you start your kick early enough to get this much extension. Remember, your bent knee moving towards uke will have force pushing against it, keeping the knee contracted, and there is a small, yet considerable amount of time for it to overcome that force and begin extension.