An easy way to remember how to calculate a point score, for the numerical levels, is to multiply the level by 6 and add 3. For example, to work out a point score for level 4 you first multilpy 4 by 6 to get 24 and add 3 to get 27.
Chris achieved a level 2b in Reading, a level 1 in Writing and a level 3 in Mathematics in the Key Stage 1 assessments.
The equivalent point scores can be identified using the table above:
2b = 15
1 = 9
3 = 21
We now add the point scores together to get the total point score: 15 + 9 + 21 = 45
Dividing the total point score by the number of pupils gives us the average point score for Chris: 45/3 = 15.0
The same methodology can be applied to a cohort of children (e.g. Free School Meals), a particular subject or the entire National Curriculum year - you still work out the equivalent point scores, add them up and then divide by the number of subjects and/or children.