Well we can't be 100% sure what their percentage was since ethanol evaporates and only the residue is left behind, telling us only what the ingredients were (with lots of expensive chemical testing). We do know that brewing techniques didn't change much until the industrial revolution and even then it only changed for larger scale operations. When making a beer like product, a high abv would be about 5-6% without using modern high yield yeast strains. That's also a longer fermentation time though, something like 4-6 days. In many ancient cultures fermentation was more like a 24 hour process, giving an expected abv of about 3%, if that. With mead and other "fruit wines", you could expect a somewhat higher average yield since it was a 100% sugar base to begin with and therefore the yeast had more to work with. A modern wine with modern production techniques usually clocks in at about 12-14% abv. A good mead maker can certainly hit the 10-12% range today and honestly, mead making is still done in very traditional styles. I would expect an accomplished mead maker in the viking age to be able to do so as well. If you want to learn more I would recommend How to make mead like a Viking by Jereme Zimmerman. He uses a completely organic, natural process that is as close as possible to ancient texhinques. He also has a variety of very herbal, traditional recipes that would be more akin to what early Nordic cultures were actually drinking.