Even if your figures were correct (I'll come to that) 60% of a small thing can be a lot less than 10% of a large thing.
Now, your figures. Could you back them up, please. Because
this article with its graph claim that about 42% of our exports go to the EU, and that this figure has been declining for about 15 years (from a high of around 55% [60-63% high in 2000 & 2006 in other studies]).
This study looks at the trade deficit with the EU. We import substantially more from the EU than we export to the EU. Amongst other things, it says that 22% of EU exports come to Britain, contradicting your claim of 10%.
It does, however, say that the balance of power in the discussion over Brexit would lie with the EU, and that may well be the case. However, if you think that the EU wouldn't be concerned with losing trade with one of its largest export markets, then you may have an argument that I haven't yet heard. I'd be interested.
You may not have taken account of a Brexit leaving the EU much reduced. We are one of its largest sources of income, and as an organisation it would have a smaller just-about-everything (not just budget) if we left. It is already in decline. There are serious structural issues with the Eurozone, and my view is that its economy has been stagnant for many years. You may differ. I suggest that if we weren't already a member but were considering joining, these two issues would be top of the agenda. You may think that a weakened and declining EU would hold all the aces during exit negotiations, but I happen to think it rather more marginal than that, as I've explained.