This is a sample sentence for 八九不离十, from 现代汉语规范词典:
不猜便罢, 若猜便猜它个八九不离十。
It uses an interesting idiom 八九不离十 which I just learnt thanks to duchinese.net.
http://www.duchinese.net/lessons/E269-eight-and-nine-are-close-to-ten
这句话里面的便是不是相当于就?但是意思还不清楚,请解释。 I think 便 functions like 就 here. But I'm not clear about the meaning. Could it be this, in English?
If you don't guess then you stop; if you do guess then get it right.