Half a chimney is more than sufficient in a barrel that size to get 250 degreesF, in fact I think your largest challenge would be to keep the temperature low enough when it's hot outside with a full load of charcoal.
It sounds like you might want to do a trial smoke with it, no meat, and see how long you can keep it at 250. I'd start with a 50% capacity, and full open all vents.
Once you hit your maximum temperature, which I'm sure will be greater than 250, begin closing the inlet vent little by little until the temperature levels off at 250. As the cook goes on, you'll want a little more air to get the remaining volume of charcoal hotter than it was, so begin opening the inlet vent to keep it at 250. Then practice adding new charcoal.
I generally don't mess with the exhaust vent, and keep it full to half open depending on the external temperature.
Make sure you have a reliable digital thermometer to monitor the temp.