When buying line, any line, if you work on the assumption that the actual breaking strain will be at least 30% higher than the one stated you won't be far out, the higher the stated breaking strain the higher the percentage too.
There is a shop in Dartford,
The Tackle Box, that send out a brochurecontaining nearly all the actual breaking strains of modern lines used by anglers and a real eye opener it is too. I had it once and sent out photo copies to Graham Marsden, Andy Nellist, Mark Wintleand some others I can't remember them all, Andy may remember because he had seen it before he told me.
They are incidentally a great place for reel repairs too and well worth remembering.