That's a lot of questions in one post!!
If you read some of the threads you will pick up some answers.
You can't work out 220 as it depends on other children in the county's performance ie it is standardised. Guidance seems to suggest a pass mark of about 75-80%, sometimes higher depending on paper. The harder the paper, the lower the pass mark due to standardisation.
I would recommend the GL assessment papers as I know the NVR used as a mock in Lincolnshire schools is one of theirs.
Don't suppose it matters too much. In VR, there always seems to be something they have not seen before. The trick is to get them to be calm and not freak out!
In fact, calmness is the key to it all in my opinion.