60% sounds really good and achievable
I'm not sure you can draw this conclusion unless you have somehow seen a copy of the real actual Bucks papers sat this year (which shouldn't be available). You really can't compare the scores with any other practice paper that's available from anywhere. Certainly not the same as Bond type.
And you really shouldn't work to 60% as a target for your child. Apart from the fact that there are no similar papers to practise on, next year's CEM paper could have pass mark nearer 40% or up to 80%, it will depend entirely on the difficulty of next year's paper and how the children find it and the level of the children sitting it.
Don't kid yourself - these were very tough papers and there are plenty of very bright kids already at level 5 in their SATs who didn't pass and some who came out below average compared to the bright cohort who take the test. They were very, very time pressured, most children will not have finished a lot of the test, and the content of the test is subtly hard. It's a real differentiator of bright kids. Hard to know if it hit it right or not... We'll probably know more in two years time after the current lot have completed year 7!