The dispensation of grace has less to do with it starting at a specific moment in time, and more to do with the proclamation of a new salvation message from God. It's not so much linked to the event of Pentecost in Acts 2, or the conversion of Paul in Acts 9, or him turning to the Gentiles in Acts 28, and more to do with receiving a message of God's abundant grace without the need of works and endurance. I'm not declining any reasoning that the message of grace started being proclaimed somewhere in the transitional book of ACTS, but I also hold true the point that grace started in each person individually when they hear, and received the good news that God declares them justified through the absolute and completed cross work of His Son, Jesus Christ.