For 100W ERP you can either put in the actual power and the antenna gain, or the 100W ERP and 0dBi... seems to make no difference, except that for a directional antenna, the hazard is significantly less in some other direction.
You should figure out the average power correctly. If FM, the average power and the PEP power are the same, so 100W is appropriate, and you're not in compliance with someone 1 meter away in an uncontrolled environment, but you are in compliance in a controlled (i.e. you or another ham who realizes that RF is being emitted any time it is on and so is "controlling" their own exposure) environment.
As far as "dwell time" goes, it's on the 16th page of this PDF file:http://www.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet65/oet65.pdf
I don't think you have much to worry about regulation-wise unless people who don't know about the RF are going to be that close to the antenna.