Make sure you place a stick with distance markers on it next to the slide or use a markerpen and jot down the height in increments on the slide itself. And preferrably also mount a high-speed camera if you have one available so we can correctly measure the falling distance.
On a more serious note you could just print out the calculations from this thread, bring it to the park and then show it to any mothers who are worried about kids falling on their bums.