Rely less on your average user to 'simply' change a mathematic formula they have no clue of understanding. That's your job.

x*(1-stark)+((len / 2-1-x)*x / (len / 2))*stark may be a simple formula to a programmer,
but definitely not to your average software user (who I believe you are marketing it to).
If it's being marketed to programmers or mathematicians feel free to ignore this suggestion.