Hrqls: That's the pure mathematical cutoff, however you do gain a slight advantage in accepting so the real number is often slightly less (depending on the match score).

In this case the 28% is your chance of winning the match after rejecting the double. In this case, because accepting basically makes this game the final one, 28% becomes the cutoff.

It seems that the chance of winning this game turned out to be 26.8% or 27.2%, so to say that it would have been a major blunder is a little harsh (unless I am missing something).