I see.
So if it want it to be 100 and have no problems with people under 100 posts I would have to set it to 99 but would still have the issue with those over 100 that only post in that forum?

The people that only posted there would have a hard time guessing they have to post somewhere else first... this is a very bad idea as they will feel resctricted even if they are not... they may even leave the community feeling unwelcome and never figure out they are the one we wanted posting...