In Back to the Future, there is a flyer indicating that the lightning struck the clock tower at 10:04pm on November 12, 1955. Based on that information, Doc calculates the distance, acceleration speed and time needed for the DeLorean to reach the cable.
But we all know that the minute has 60 seconds. And the lightning could strike any second within that minute. How could Doc predict that it would be at exactly 10:04:00? And thus he was risking. Should Marty arrive 2 seconds later or sooner, the experiment would fail.