Look Back

[DeadNoMore]

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