Right, here we go. The reason why you are getting a negative figure is because your target memory is greater than the actual total server memory. In other words your sql server is configured with an inaccurate memory and not dynamic or the server is under resourced. Every sql server is different so it is pointless comparing to others as the databases will never be the same except with replication on clusters. I would check the server memory resources configured on the sql server. The negative figure needs to be added to the total memory to get an indication of the correct total memory that should be assigned.
Hope this helps.