Look Back

I am trying to get my data so on the first of the month, it looks back for any entry in a particular column "Points". I need it to check and if there are no entries for the prevoius two calendar months return a value of -1. Then I need it to wait two months and do it again, and so on for 12 months. If when it looks back and finds an entry, I need the process to start all over again. Each two months with no entry will render a -1 until it reachs 0.

Basically, two months with no entry is reward of -1. if it goes anther two months with no entry another reward of -1. etc. The Point will be deducted from a twelve month total until it hits zero. It can't go below zero. No negative total.

I figure I will need to have the twelve month total - the reward column = total points but total points can't ever be a negative number.

I'm new to BI so I'm not great at coding. Any help would be appreciated.

Thanks