Depends how much detail you want
VR has twice the weighting of the maths and the english.
Further adjustment to allow for how difficult the particular papers are.
The final figure is then used to rank all the entrants.
There is no adjustment in Essex re age of entrant.
It seems complicated but I'm not sure its that important. Its mainly a statistical process.
The main point in terms of preparing your child is that VR is the most important paper - if you do well on that you can afford to do less well on the other two. Whereas dropping marks on VR can 'hit hard'.
Do look at previous threads - will give you realistic idea of the marks needed for different schools. Not as high as some people would lead you to believe but most children do score lower in the 'real thing' than at home.
Hope this is what you needed.