This is where some confusion comes in with the current system. According to the Wiki it is your "Turning Value" plus the bonus based on the D20 roll and your
Charisma modifier (IE:
Turn Undead Check Bonus). My understanding then, is that the current formula for
Turning Value is:
Cleric Level +
Turn Level Bonus +
MaxHD Bonus and that your
Turn Undead Check Bonus will be added to this to determine your
Total Turning Value. Your
Total Turning Value must be greater than the Hit Dice of an Undead Mob to successfully "turn" it at all and must be DOUBLE the Hit Dice of an Undead Mob in order to instantly destroy it. (With
Mighty Turning if the Total Turning Value is greater than or equal to the Hit Dice of the mob then it is destroyed.)