the most efficient way, is to send out your drone when you make your first overlord. The reason for this: the timming of your first overlord determines what kind of build you are doing also determining what kind of pressure you are able to handle. example: if you were to 10pool, no point in sending your drone at 9, because if you're opponent is rushing, you will be prepared since you 10pooled. if you were to 14pool or 14hatch, you would get your overlord at 9, in order to keep up a strong economy meaning you are more vulnerable, so sending your drone at 9 gives you enough time to see if your opponent is rushing so you know you have to put up some spine crawlers or zerglings. you might be asking whats the difference? its still 9 or 10 in both my scenarios.. but when I play zerg, i use my extractor trick, so i get my overlord at 12, so i send my drone at 12, therefore whether or not he is rushing, ill have at least 8 lings out, i only want to see what he is teching up to or just do some damage.