Rennet can be variable in strength even when it comes from the same manufacturer and is labeled the same, so it always is important to adjust for actual results with any given batch of rennet. It sounds to me like you have some that is super concentrated. I would try using half as much the next time, and see how it goes; if still too fast, use half as much the time after that; and so on. Of course, it will get harder and harder to measure the smaller and smaller amounts. One answer is to make a larger batch of cheese - instead of using half as much rennet, use twice as much milk! Of course, that can run into problems with appropriate sized moulds and pressing weights and so on.