What is the formula for calculating constant percent change?

For the period from ##t_1## to ##t_2## for some state ##f(t)## with a constant percentage change that constant percentage change is given by:
##color(white)(XX)(f(t_2)/f(t_1)-1)*100%##
The actual change from ##t_1## to ##t_2## is
##color(white)(XX)f(t_2)-f(t_1)##
The change as a fraction of its value at ##t_1## is
##(f(t_2)-f(t_1))/f(t_1) color(white)(XX)=color(white)(XX) f(t_2)/f(t_1) -1##
or
as a percentage:
##color(white)(XX)(f(t_2)/f(t_1)-1)*100%##