First...great job on the first time out.
My suggestion is rather than counting out the briquettes, get yourself a full size weber chimney. The full size has the extra metal swing handle to assist with dumping coals.
Most guides, tips, etc are designed around "full chimney" or "half chimney" suggestions.
The Sns works extremely well at maintaining a nice steady 350 for chicken with a full chimney and 1/2-3/4 bottom vent.