What should I bet to protect a made straight from a flush draw?

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A pot raise would be enough to force him to make a mistake. Just add up the pot up to your decision plus the amount you are about to call and add that sum to the amount you are about to call- that's the amount you should raise to.

That would be: 3.25+6+6+3+3=$21.25

You only raised to $10, which allowed UTG+1 to call 7 to win a pot of $25.25 with MP2 left to act who will probably call the raise and add to the pot. So he had 22% to call and a 20% to win, but the -2% is made up for the implied odds on the River as someone will surely pay him off so he pretty much did not make a mistake, as he also had a straight on the Turn.

Another reason why I love playing at Ultimate Bet- they already have the pot amount automatically calculated for you, saving you lots of headaches.