Ya it absolutely does not have to work 2/3 of the time. Say you bet 20$ into a 30$ pot. You are getting 2:5 on your money, so the bet only has to work 40% of the time. If you bet 15$ into a 30$ pot, it only has to work 1/3 of the time, because you are getting 1:3 on your money.
And that is why i think you should c-bet. As i said before, i think he is only really likely to have 4 or 5 sort of hands (99-QQ or AK). The odds
that he does not have JJ/QQ (the only hands he will call your c-bet out of those) is probably no greater than 50%. In fact, firing a little over 1/2 the pot is probaby the best option in this case. Any hand that will fold to a c-bet, isn't likely to call under any circumstances (99-TT, AK) and any other hand will call/reraise...so you should minimize your losses.
Except that if he calls the c-bet, you are done. You cannot fire a 2nd barrel at that flop under any circumstances unless you hit your straight or maybe TPTK (but that is probably still beat). So, any hand you are going to chase off for 40$, you will chase off for far less, and any other hand has you dominated and you are just throwing money away. The ONLY hands that a high cbet will be useful against are AQ and AJ, and i personally don't see those hands calling a 3 bet, since they could so easily be outkicked and dominated (but i could be wrong on this account coz i don't play NL100).
Obviously the key in this situation is to figure out how little can you bet, but still get him to lay down the hands that have you beat (or tied), but not dominated.