You only hit the flop 33% of the time

40%  C-Bet = 33/40 = 82.5% of time hitting the flop therefore people will fold

60% C-Bet = 33/60 = 55% of time hitting the flop therefore 45% of  time missing

75% C-Bet = 33/75 = 44% of time hitting the flop therefore 56% of time missing

>80%  you can be exploited if your

If turn C-Bet is 30-33% it means this is where he becomes honest as you hit the flop 33% of the time