since doing calculations for semibluffs on the fly seems somewhat demanding i have made myself a chart for some common values. the values here though do not take into account $ amounts to be called. needed fold percentages would have to be somewhat higher if your semibluff is a reraise. for a plain raise though they may serve as a quickreference.

http://img174.imageshack.us/img174/6290/fvshvbw7.jpg
Fold% = breakeven point
Handvalue% = chance of winning hand when called

cheers