I'm making it too complicated?

I'm just trying to work out what people mean.

So you are saying that roughly 30% of children nationally get level 5b (or is it 5c) or higher at the end of KS2, and these are the children that an 11plus test for a county like Bucks should be trying to pick out.

So in Bucks, for a test at the beginning of year 6 you should be trying to pick out children who would have been high 4s at the end of year 5. Then by looking at the archived NNS and the units and differentiating the year 5 work described within that, we have identified the material that a child should have covered by the end of year 5 in maths to have a reasonable shot at the new test in Bucks if this is what CEM have been told to based it on.

Please tell me one more thing - does "differentiating" the year 5 NNS units for the top 30% of the year 5 cohort mean covering those units to the their "ultimate limit" or adding in a touch of year 6 work?

Yes I know that the same topics come up again and again in maths and you can take each topic " a bit further" ... but when does a bit further start being the next year's work and therefore not part of the thing I was trying to define at the beginning? Maths repeats itself for many school years. So do a lot of subjects.