Hello,
Great question, I am trying to resolve that issue myself.
When to semi-
bluff.
I have no problem understand the technique. My problem is courage. It is difficult to hammer at a pot when you are deep into a tournament and the semi-bluff move is going to take 30% of your stack. However, if it works.....you are in phat city!!! Having the guts to pull the trigger is what I need to work on.
However enough about me....to answer your question it really boils down to how well you know your opponent. Is he a loose-passive player that will call you down to the river? A tight-agressive opponent? Tight-weak? You have to know where your opponent is capable of doing.
Other issues you need to consider: do you have position on your opponent? If this is a tourny, how deep are you in? What is the relation between the blinds/ante's on your stack and what is the relation between the blind/ante's on your
opponents stack.
If all those conditions are favorable then push in a bet of at least 1/2 the size of the pot. You might even consider a pot size bet.
There is no easy answer to your question. Hopefully some folks here can give us more input.