I firmly believe in safe diving and think computers make me safer. But it is good to keep in mind that the numbers you see are approximations and subject to some variability. It is the built in safety margins that make this ok. That got driven home on a recent dive where I was coming up the anchor line, then across another line, and heading to the hang bar under the boat. In doing so I was between 15 and 20 ft for a bit and my hand was elevated at one point holding a line. I dive two scubapro computers on my left arm. They start the safety stop countdown at 15 ft and continue as long as I am above 20 ft. I looked at my computers and on one of them I had just cleared my safety stop and on the other I was just starting and it showed me with 2:54 to go. The computers are about 3 inches apart on my arm.