Business Rule - Sum of all records

[louisestern]

I have created a custom mdf for driving offences which records the history of all Company car driver offences including if they recieved points on thier license.

I want to get a cumulative total if say they got 3 points 2 years ago and now recieved another 3 points i want it to populate the total in the current record. I then also want to set an alert once rhis hits 6 points or over however, i cant work out how to create a rule to add the "points recieved" field together.

Can anyone offer some guidance?

This is my parent object:

This is my child object:

These are the relevant fields