This is a sample sentence for 八九不离十, from 现代汉语规范词典：
It uses an interesting idiom 八九不离十 which I just learnt thanks to duchinese.net.
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.