Nollie and fakie 180s are different from normal ones because your momentum is moving towards your popping foot, so like rather than winding up and rotating at the same time as popping and using your momentum to guide your back foot around, for nollie and fakie your upper body needs to rotate just before the pop, so that your lower body can follow it on the way up, and when its done well the peak of the 180 will be at 90 degrees and the last 90 will be on the way down. Because your popping from in front of you, the same direction your momentum is going, I think the biggest thing to pay attention to is balancing how quickly or aggressively you swing your shoulders around with how much power you put into popping, and timing them properly, once the height of your 180s is at its peak when your 90 degrees through it consistently you can start trying to bone them out on the way down, I think at about 80 degrees is where you want to start kicking forward with your front foot