Question-and-Answer Resource for the Building Energy Modeling Community
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First, if I understand correctly the heating system you are describing, this is merely a baseboard. A baseboard is typical a pipe with fins to enhance the heat transfer by having a greater surface area, but not necessarily: if you have enough piping itself there's no need for fins.
Second, I'd suggest you calibrate first on gas usage (energy) and not peak (demand). Make sure that you can get close enough, that will help you tune the assumptions you had to take (occupancy, actual R-values, average thermostat setpoints, infiltration levels etc).
Then you can consider trying to further refine to approximate the peak. But note that it will require a much deeper granularity in your modeling. You really have to understand how the heating system as a whole is controlled and translate that properly. For example it's likely that your hourly peak value could be on a cold morning if the building has temperature setbacks and "boost" mode to bring back the building at the right temperature.
