What is the function of 便 here? And what does the sentence mean? 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.
不猜便罢, 若猜便猜它个八九不离十


Yes, 便 here means 就. The whole sentence means 要么就不猜，要是猜的话就会猜个八九不离十. In English, it could be like: Either they don't guess it, or they do - and they'll probably get it right.