Hi Mercedes,
For the floc method, you need a small bowl. I use a small snap lid plastic container about the size of my fist. Anyway, sterilise it, and when you add your rennet, float the bowl on top of the milk. If you tap it lightly, it will float around on the surface. The floc point is when the bowl won't move when you tap it. As you're getting close, the bowl will skid a short distance, but at floc it sticks there.
Now, let's say that is 15 minutes (as per your example). If you are using a 2x multiplier, then that would equal 30 minutes. You've already waited 15, so you just wait 15 more minutes. The time you wait is the time since you added the rennet, not additional time since floc.
You then cut at that point in time, and don't worry whether or not you have a clean break.
Using this method, you want to adjust the amount of rennet you use to target a floc time in the 10-15 minute range. So if you are reacing floc at 5 minutes, cut your rennet in half, etc.
- Jeff