The Altitude Drop Before Parachute Opens formula `(y = b - c(t + ke^(-t"/"k)))` computes the altitude of person based on their initial jump altitude (ja) and the duration of their fall prior to opening their parachute.
Caution: This formula is from problem 3 in chapter 2 on page 89 of Light and Matter. Neither the author nor vCalc asserts the accuracy of this equation at this time.