The rate is 4.56 per $1000 in sales price. So if the property were to sell for $100,000. The amount would be $456.
The stamps tax is due at the time of the closed sales transaction to the state (payable at the registry - in the case of a Milton Mass sale it would be the Norfolk Registry).
The stamps tax is based upon the gross sales price. This means that if the seller pays some closing costs for the buyer in the form of a credit which reduces the sales price, the seller pays on the above the line figure.