The town has calculated that approving the Proposition 2½ override on the Sept. 10 ballot would add 31 cents for every $1,000 of a home’s assessed value to the annual property tax bill, according to Town Treasurer Jean Kopke. That translates to about $100 more on the average residential tax bill, she said. The override would add 63 cents per $1,000 of a business’s assessed property value, Kopke said. The ballot question asks voters to allow the town to raise taxes by $320,000 to pay for four more firefighters. The measure was approved by Town Meeting but needs a majority of a townwide vote as well, Kopke said.