Now I am thinking it may be easier to scale back that idea and instead use the threat rating as a one for one measure of the goblin hordes size. So each point of threat yields more goblins to the horde, be it in the form of attracted allies, conquered tribes or birth. Of your goblins 1/3 are going to be warriors with the rest representing non combatant goblins, the chief and his guard.
There is going to a need to house these goblins too. I am going to work on the locations I have mentioned in the story points and come up with a maximum occupancy number for each. The warren at the tree will only old 75 goblins but the old Dwarven hold may allow 150, for example. There may be a need to split the tribe at times perhaps deploying goblins to hold locations or having specially trained goblins make a colony at various locations. There is going to be a mechanism for creating specialized troops of goblins.
My plan is to use the stealth, archery and combat ratings granted in story points to allow the players to unlock different types of goblin units. So if the Horde has a high magic rating they will have access to casters or shamans. Stealth could allow for the training of sneaks and assassins. These units will come into play when larger conflicts come to the horde and a more Battle System like combat is used.
Still I have not completely disregarded the chart idea. I am thinking there should be some events that are triggered by the horde size reaching a certain point. I know at 300 I want to have a group of stone giants offer to build or expand some of the goblins structures. Later on they will try to kick out the goblins and take the structure for themselves using hidden passages, but that is another story point.