Don't get too caught up with stretching between 1st and 2nd crack...especially in a popper which is unmodified it is a little difficult to control. In some instances I found stretching can actually detract from the roast taste and can in fact bake the beans so that you've lost much of the desired characteristics of the beans. My only recommendation would be to do two or three roasts on the same day, one where you just let the popper run its course, and the next where you stretch it a little and the last where you do what you've been doing with your 15 mins roast. label each roast and after each passing day, smell, taste and note down the differences. From there you will know what profile you like for that particular bean.