Here is teamrankings.com's PACE formula:
The _1 suffix means the value is for the team in question. Th _2 suffix means the value is for the opponent(s). All SUM() or AVG() are taken over all games of a season.
Possessions per Game<O</O
AVG(((fga_1 - oreb_1 + turnovers_1 + (0.475 * fta_1)) + (fga_2 - oreb_2 + turnovers_2 + (0.475 * fta_2))) * 0.5)<O</O