I have found first crack hard to hear with a lot of beans mate and what you thought was first crack was more than likely 2nd crack.
Try P2 as it profiles better and is not as harsh as P1.
P2 will taper off during the roast. *You want to try and get to 1st crack around the time of the temp drop and let the beans coast to 2nd crack. *Watch out though as the profile ramps back up to 100% power for the last bit of the roast and you want to manually stop the roast before this time.
So just play around with the starting weight to achieve these goals, a few trial and error roasts later and you will get the hand of it pretty quick.
Use the search function though as there are a couple of threads that will explain most of what you need to know.