Fix index off by one error for flux percential ratio

[stevenstetzler]

Here's the PR. Perhaps it would be good to rename

lc_length = len(sorted_data) - 1

F_60_index = int(math.ceil(0.60 * lc_length))
...

to

lc_length = len(sorted_data)
lc_index = lc_length - 1

F_60_index = int(math.ceil(0.60 * lc_index))
...

or to

lc_index = len(sorted_data) - 1

F_60_index = int(math.ceil(0.60 * lc_index))
...

to keep the semantics of the variable lc_length consistent.