3e+3 = 3 * 10^+3 = 3000
3e-3 = 3 _ 10^-3 = 3 _ 1/10^3 = 0.003
math is elegate precision instead of sharp
((To - From) / From)
With a value "from" of 5.7s and "to" of 2.1s
2.1 - 5.7 = -3.6
3.6 / 5.7 = -0.63157895
# format to percent
0.63157895 * 100 = -63.157895 %
Rounding to two decimal places would yield a decrease in the speed index of 63.16%.