1 - ((m-20) / 34)

results = 1 - ((34-20) / 34) == 0.588

$ m=34
$ bc <<< "scale = 10; 1 - (($m - 20) / 34)"
.5882352942

$ bc -l <<< "1 - (($m - 20) / 34)"
.58823529411764705883

printf "%.3f\n" $(bc -l ...)